diff --git a/README.md b/README.md
index e6932f9..d4068e1 100644
--- a/README.md
+++ b/README.md
@@ -276,6 +276,11 @@ Configure via `require("peekstack").setup({ ... })`.
 
 </details>
 
+> [!NOTE]
+> `setup()` never throws on bad config. Invalid values fall back to defaults
+> with a `vim.notify` warning, and unknown or mistyped keys (e.g. `ui.popups`
+> instead of `ui.popup`) are reported the same way so typos are easy to spot.
+
 ## 🧺 Picker backends (telescope / fzf-lua / snacks.nvim)
 
 peekstack uses a picker when multiple locations are returned (e.g. references). The
diff --git a/lua/peekstack/config/validate/init.lua b/lua/peekstack/config/validate/init.lua
index 14ec46a..df35e34 100644
--- a/lua/peekstack/config/validate/init.lua
+++ b/lua/peekstack/config/validate/init.lua
@@ -2,12 +2,16 @@ local ui = require("peekstack.config.validate.rules.ui")
 local picker = require("peekstack.config.validate.rules.picker")
 local providers = require("peekstack.config.validate.rules.providers")
 local persist = require("peekstack.config.validate.rules.persist")
+local unknown = require("peekstack.config.validate.unknown")
 
 local M = {}
 
 ---@param cfg table
 ---@param defaults PeekstackConfig
 function M.run(cfg, defaults)
+  -- Detect unknown keys first, before field validators can replace invalid
+  -- subtrees with defaults (which would hide sibling typos).
+  unknown.detect(cfg, defaults)
   ui.validate(cfg, defaults)
   picker.validate(cfg, defaults)
   providers.validate(cfg, defaults)
diff --git a/lua/peekstack/config/validate/unknown.lua b/lua/peekstack/config/validate/unknown.lua
new file mode 100644
index 0000000..0a2019f
--- /dev/null
+++ b/lua/peekstack/config/validate/unknown.lua
@@ -0,0 +1,64 @@
+local notify = require("peekstack.util.notify")
+
+local M = {}
+
+-- Subtrees keyed by user-defined names (provider names, filetypes) rather than
+-- a fixed schema. Unknown-key detection must not descend into these.
+---@type table<string, boolean>
+local OPEN_PATHS = {
+  ["ui.title.icons.map"] = true,
+  ["ui.title.context.node_types"] = true,
+}
+
+---@param defaults table
+---@return boolean True when the table is a fixed record (not a list/array).
+local function is_record(defaults)
+  return not vim.islist(defaults) and next(defaults) ~= nil
+end
+
+---@param defaults table
+---@return string[] Sorted known keys at this level.
+local function known_keys_of(defaults)
+  local keys = {}
+  for key in pairs(defaults) do
+    keys[#keys + 1] = tostring(key)
+  end
+  table.sort(keys)
+  return keys
+end
+
+---@param section table Merged config subtree.
+---@param defaults table Default schema subtree.
+---@param prefix string Dotted path of this subtree ("" at the root).
+local function walk(section, defaults, prefix)
+  for key, value in pairs(section) do
+    local path = prefix == "" and tostring(key) or (prefix .. "." .. tostring(key))
+    if defaults[key] == nil then
+      notify.warn(
+        string.format(
+          "Unknown config key %q (ignored). Known keys: %s",
+          path,
+          table.concat(known_keys_of(defaults), ", ")
+        )
+      )
+    elseif
+      type(value) == "table"
+      and type(defaults[key]) == "table"
+      and is_record(defaults[key])
+      and not OPEN_PATHS[path]
+    then
+      walk(value, defaults[key], path)
+    end
+  end
+end
+
+---Warn about unknown/typo'd config keys (e.g. `ui.popups` instead of
+---`ui.popup`). Compares the merged config against the default schema and
+---reports keys the user supplied that peekstack does not recognise.
+---@param cfg table Merged config.
+---@param defaults PeekstackConfig
+function M.detect(cfg, defaults)
+  walk(cfg, defaults, "")
+end
+
+return M
diff --git a/lua/peekstack/ui/diagnostics.lua b/lua/peekstack/ui/diagnostics.lua
index dd03384..5e75167 100644
--- a/lua/peekstack/ui/diagnostics.lua
+++ b/lua/peekstack/ui/diagnostics.lua
@@ -87,21 +87,28 @@ function M.decorate(popup)
   end
 
   if #virt_lines > 0 then
-    local id = vim.api.nvim_buf_set_extmark(bufnr, NS, line, 0, {
+    -- pcall guards against out-of-range coordinates raised by the API.
+    local ok, id = pcall(vim.api.nvim_buf_set_extmark, bufnr, NS, line, 0, {
       virt_lines = virt_lines,
       virt_lines_above = true,
     })
-    table.insert(ids, id)
+    if ok then
+      table.insert(ids, id)
+    end
   end
 
   local underline = severity_hl(location.kind, "DiagnosticUnderline")
   if underline ~= "" then
-    local id = vim.api.nvim_buf_set_extmark(bufnr, NS, line, col, {
+    -- end_col may exceed the line length when the popup buffer is truncated;
+    -- pcall keeps decoration best-effort instead of erroring on the whole popup.
+    local ok, id = pcall(vim.api.nvim_buf_set_extmark, bufnr, NS, line, col, {
       end_row = end_line,
       end_col = end_col,
       hl_group = underline,
     })
-    table.insert(ids, id)
+    if ok then
+      table.insert(ids, id)
+    end
   end
 
   if #ids == 0 then
diff --git a/lua/peekstack/ui/feedback.lua b/lua/peekstack/ui/feedback.lua
index b9c2cba..3aa8596 100644
--- a/lua/peekstack/ui/feedback.lua
+++ b/lua/peekstack/ui/feedback.lua
@@ -16,8 +16,14 @@ function M.highlight_origin(origin)
   if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
     return
   end
-  local row = math.max((origin.row or 1) - 1, 0)
-  vim.api.nvim_buf_set_extmark(bufnr, ns, row, 0, {
+  local line_count = vim.api.nvim_buf_line_count(bufnr)
+  if line_count == 0 then
+    return
+  end
+  -- Clamp the origin row in case the buffer shrank since the popup opened.
+  local row = math.min(math.max((origin.row or 1) - 1, 0), line_count - 1)
+  -- Guard against transient API failures (e.g. concurrent buffer edits).
+  pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, row, 0, {
     end_row = row + 1,
     hl_group = "PeekstackOrigin",
   })
diff --git a/lua/peekstack/ui/stack_view/init.lua b/lua/peekstack/ui/stack_view/init.lua
index 88de44f..cc2bbe0 100644
--- a/lua/peekstack/ui/stack_view/init.lua
+++ b/lua/peekstack/ui/stack_view/init.lua
@@ -54,7 +54,7 @@ function M.open()
       keymaps.close_help(s, opts, keymap_deps())
     end,
     ensure_non_header_cursor = function()
-      keymaps.ensure_non_header_cursor(s)
+      state.ensure_non_header_cursor(s)
     end,
   })
 
diff --git a/lua/peekstack/ui/stack_view/keymaps.lua b/lua/peekstack/ui/stack_view/keymaps.lua
index c30f11f..4e7ee32 100644
--- a/lua/peekstack/ui/stack_view/keymaps.lua
+++ b/lua/peekstack/ui/stack_view/keymaps.lua
@@ -3,6 +3,7 @@ local location = require("peekstack.core.location")
 local str = require("peekstack.util.str")
 local notify = require("peekstack.util.notify")
 local keymap_spec = require("peekstack.ui.keymap_spec")
+local state = require("peekstack.ui.stack_view.state")
 
 local M = {}
 
@@ -57,24 +58,6 @@ local function entry_lines(s)
   return lines
 end
 
----@param s PeekstackStackViewState
-function M.ensure_non_header_cursor(s)
-  if not (s.winid and vim.api.nvim_win_is_valid(s.winid) and s.bufnr and vim.api.nvim_buf_is_valid(s.bufnr)) then
-    return
-  end
-
-  local line_count = vim.api.nvim_buf_line_count(s.bufnr)
-  if line_count <= 0 then
-    return
-  end
-
-  local min_line = math.min((s.header_lines or 0) + 1, line_count)
-  local cursor = vim.api.nvim_win_get_cursor(s.winid)[1]
-  if cursor < min_line then
-    vim.api.nvim_win_set_cursor(s.winid, { min_line, 0 })
-  end
-end
-
 ---@param s PeekstackStackViewState
 ---@param step integer
 local function move_cursor_by_stack_item(s, step)
@@ -84,7 +67,7 @@ local function move_cursor_by_stack_item(s, step)
 
   local lines = entry_lines(s)
   if #lines == 0 then
-    M.ensure_non_header_cursor(s)
+    state.ensure_non_header_cursor(s)
     return
   end
 
@@ -364,7 +347,7 @@ local function build_specs(s, deps)
       rhs = function()
         local lines = entry_lines(s)
         if #lines == 0 then
-          M.ensure_non_header_cursor(s)
+          state.ensure_non_header_cursor(s)
           return
         end
         vim.api.nvim_win_set_cursor(s.winid, { lines[1], 0 })
@@ -375,7 +358,7 @@ local function build_specs(s, deps)
       rhs = function()
         local lines = entry_lines(s)
         if #lines == 0 then
-          M.ensure_non_header_cursor(s)
+          state.ensure_non_header_cursor(s)
           return
         end
         vim.api.nvim_win_set_cursor(s.winid, { lines[#lines], 0 })
diff --git a/lua/peekstack/ui/stack_view/render.lua b/lua/peekstack/ui/stack_view/render.lua
index c30f3ea..6d2e077 100644
--- a/lua/peekstack/ui/stack_view/render.lua
+++ b/lua/peekstack/ui/stack_view/render.lua
@@ -2,27 +2,10 @@ local config = require("peekstack.config")
 local location = require("peekstack.core.location")
 local diff = require("peekstack.ui.stack_view.diff")
 local pipeline = require("peekstack.ui.stack_view.pipeline")
+local state = require("peekstack.ui.stack_view.state")
 
 local M = {}
 
----@param s PeekstackStackViewState
-local function ensure_non_header_cursor(s)
-  if not (s.winid and vim.api.nvim_win_is_valid(s.winid) and s.bufnr and vim.api.nvim_buf_is_valid(s.bufnr)) then
-    return
-  end
-
-  local line_count = vim.api.nvim_buf_line_count(s.bufnr)
-  if line_count <= s.header_lines then
-    return
-  end
-
-  local min_line = s.header_lines + 1
-  local cursor = vim.api.nvim_win_get_cursor(s.winid)[1]
-  if cursor < min_line then
-    vim.api.nvim_win_set_cursor(s.winid, { min_line, 0 })
-  end
-end
-
 ---@param s PeekstackStackViewState
 ---@param is_ready fun(s: PeekstackStackViewState): boolean
 function M.render(s, is_ready)
@@ -59,7 +42,7 @@ function M.render(s, is_ready)
   s.line_to_id = model.line_to_id
   s.header_lines = model.header_lines
   s.render_keys = diff.apply(s.bufnr, s.render_keys or {}, model, s.preview_ts_cache)
-  ensure_non_header_cursor(s)
+  state.ensure_non_header_cursor(s)
 end
 
 return M
diff --git a/lua/peekstack/ui/stack_view/state.lua b/lua/peekstack/ui/stack_view/state.lua
index 67cbc43..ea2d377 100644
--- a/lua/peekstack/ui/stack_view/state.lua
+++ b/lua/peekstack/ui/stack_view/state.lua
@@ -59,6 +59,27 @@ function M.reset_open_state(s)
   s.preview_ts_cache = {}
 end
 
+---Move the cursor below the header rows when it would otherwise sit on a
+---header line. No-op when the buffer holds only header lines.
+---@param s PeekstackStackViewState
+function M.ensure_non_header_cursor(s)
+  if not (s.winid and vim.api.nvim_win_is_valid(s.winid) and s.bufnr and vim.api.nvim_buf_is_valid(s.bufnr)) then
+    return
+  end
+
+  local line_count = vim.api.nvim_buf_line_count(s.bufnr)
+  local header_lines = s.header_lines or 0
+  if line_count <= header_lines then
+    return
+  end
+
+  local min_line = header_lines + 1
+  local cursor = vim.api.nvim_win_get_cursor(s.winid)[1]
+  if cursor < min_line then
+    vim.api.nvim_win_set_cursor(s.winid, { min_line, 0 })
+  end
+end
+
 local function cleanup_invalid_states()
   for tabpage, s in pairs(states) do
     if not vim.api.nvim_tabpage_is_valid(tabpage) then
diff --git a/lua/peekstack/ui/viewport.lua b/lua/peekstack/ui/viewport.lua
index aa6760a..facd2e1 100644
--- a/lua/peekstack/ui/viewport.lua
+++ b/lua/peekstack/ui/viewport.lua
@@ -45,19 +45,24 @@ function M.decorate(popup)
   local ids = {}
 
   if viewport.skipped_before and viewport.skipped_before > 0 then
-    local id = vim.api.nvim_buf_set_extmark(bufnr, NS, 0, 0, {
+    -- pcall guards against out-of-range coordinates raised by the API.
+    local ok, id = pcall(vim.api.nvim_buf_set_extmark, bufnr, NS, 0, 0, {
       virt_lines = { { { format_marker(viewport.skipped_before, "above"), "PeekstackViewportTruncated" } } },
       virt_lines_above = true,
     })
-    ids[#ids + 1] = id
+    if ok then
+      ids[#ids + 1] = id
+    end
   end
 
   if viewport.skipped_after and viewport.skipped_after > 0 then
     local last = line_count - 1
-    local id = vim.api.nvim_buf_set_extmark(bufnr, NS, last, 0, {
+    local ok, id = pcall(vim.api.nvim_buf_set_extmark, bufnr, NS, last, 0, {
       virt_lines = { { { format_marker(viewport.skipped_after, "below"), "PeekstackViewportTruncated" } } },
     })
-    ids[#ids + 1] = id
+    if ok then
+      ids[#ids + 1] = id
+    end
   end
 
   if #ids == 0 then
diff --git a/tests/config_spec.lua b/tests/config_spec.lua
index 47676a6..1c2fdea 100644
--- a/tests/config_spec.lua
+++ b/tests/config_spec.lua
@@ -778,5 +778,58 @@ describe("config", function()
       assert.is_true(has_message("persist.session must be a table"))
       assert.equals("table", type(cfg.persist.session))
     end)
+
+    describe("unknown keys", function()
+      it("warns on an unknown top-level key", function()
+        config.setup({ unknown_top = true })
+        assert.is_true(has_message("Unknown config key"))
+        assert.is_true(has_message("unknown_top"))
+      end)
+
+      it("warns on a mistyped nested section key", function()
+        config.setup({ ui = { popups = { editable = true } } })
+        assert.is_true(has_message("ui.popups"))
+      end)
+
+      it("warns on a mistyped keymap action name", function()
+        config.setup({ ui = { keys = { focus_nxt = "<C-n>" } } })
+        assert.is_true(has_message("ui.keys.focus_nxt"))
+      end)
+
+      it("does not warn for valid config", function()
+        config.setup({
+          ui = { popup = { editable = true }, keys = { close = "x" } },
+          picker = { backend = "telescope", builtin = { preview_lines = 3 } },
+          providers = { marks = { enable = true } },
+          persist = { enabled = true, auto = { enabled = true } },
+        })
+        assert.is_false(has_message("Unknown config key"))
+      end)
+
+      it("allows custom provider icons in ui.title.icons.map", function()
+        config.setup({
+          ui = { title = { icons = { map = { custom_provider = "" } } } },
+        })
+        assert.is_false(has_message("Unknown config key"))
+      end)
+
+      it("allows custom filetypes in ui.title.context.node_types", function()
+        config.setup({
+          ui = { title = { context = { node_types = { mylang = { "func" } } } } },
+        })
+        assert.is_false(has_message("Unknown config key"))
+      end)
+
+      it("does not flag extended close_events list entries", function()
+        config.setup({
+          ui = {
+            inline_preview = {
+              close_events = { "CursorMoved", "InsertEnter", "BufLeave", "WinLeave", "FocusLost" },
+            },
+          },
+        })
+        assert.is_false(has_message("Unknown config key"))
+      end)
+    end)
   end)
 end)
diff --git a/tests/diagnostics_popup_spec.lua b/tests/diagnostics_popup_spec.lua
index c689d6e..4644c96 100644
--- a/tests/diagnostics_popup_spec.lua
+++ b/tests/diagnostics_popup_spec.lua
@@ -53,6 +53,33 @@ describe("peekstack.ui.diagnostics", function()
     vim.fn.delete(tmpfile)
   end)
 
+  it("does not error when diagnostic end_col exceeds the line length", function()
+    local diagnostics = require("peekstack.ui.diagnostics")
+    local bufnr = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, { "ab" })
+
+    local model = {
+      bufnr = bufnr,
+      line_offset = 0,
+      location = {
+        provider = "diagnostics.under_cursor",
+        text = "boom",
+        kind = vim.diagnostic.severity.ERROR,
+        range = {
+          start = { line = 0, character = 0 },
+          ["end"] = { line = 0, character = 999 },
+        },
+      },
+    }
+
+    local ok, result = pcall(diagnostics.decorate, model)
+    assert.is_true(ok)
+    -- The virt_lines marker is still added even though the underline is dropped.
+    assert.is_not_nil(result)
+
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end)
+
   it("clears diagnostic extmarks on close in source mode", function()
     local tmpfile = vim.fn.tempname() .. ".lua"
     vim.fn.writefile({ "line1", "line2" }, tmpfile)
diff --git a/tests/feedback_spec.lua b/tests/feedback_spec.lua
new file mode 100644
index 0000000..eb84e8a
--- /dev/null
+++ b/tests/feedback_spec.lua
@@ -0,0 +1,42 @@
+describe("peekstack.ui.feedback", function()
+  local config = require("peekstack.config")
+  local feedback = require("peekstack.ui.feedback")
+
+  before_each(function()
+    config.setup({})
+  end)
+
+  it("highlights the origin row", function()
+    local bufnr = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, { "line1", "line2", "line3" })
+    local winid = vim.api.nvim_get_current_win()
+
+    feedback.highlight_origin({ winid = winid, bufnr = bufnr, row = 2, col = 0 })
+
+    local ns = vim.api.nvim_get_namespaces()["peekstack_origin"]
+    assert.is_not_nil(ns)
+    local marks = vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, {})
+    assert.equals(1, #marks)
+    assert.equals(1, marks[1][2]) -- row 2 (1-based) -> extmark row 1 (0-based)
+
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end)
+
+  it("does not error when the origin row is beyond the buffer", function()
+    local bufnr = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, { "only line" })
+    local winid = vim.api.nvim_get_current_win()
+
+    local ok = pcall(feedback.highlight_origin, { winid = winid, bufnr = bufnr, row = 1000, col = 0 })
+    assert.is_true(ok)
+
+    -- The row is clamped, so the highlight still lands on the last line.
+    local ns = vim.api.nvim_get_namespaces()["peekstack_origin"]
+    assert.is_not_nil(ns)
+    local marks = vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, {})
+    assert.equals(1, #marks)
+    assert.equals(0, marks[1][2])
+
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end)
+end)