diff --git a/lib/Translation/ForthToMLIR/ForthToMLIR.cpp b/lib/Translation/ForthToMLIR/ForthToMLIR.cpp
index aebb7e8..d12d2a8 100644
--- a/lib/Translation/ForthToMLIR/ForthToMLIR.cpp
+++ b/lib/Translation/ForthToMLIR/ForthToMLIR.cpp
@@ -763,7 +763,11 @@ LogicalResult ForthParser::parseBody(Value &stack) {
         builder.create<cf::BranchOp>(loc, mergeBlock, ValueRange{stack});
 
         // Pop the false-path block (from IF) - this becomes else-body start.
+        if (cfStack.empty())
+          return emitError("ELSE without matching IF");
         auto [tag, joinBlock] = cfStack.pop_back_val();
+        if (tag != CFTag::Orig)
+          return emitError("ELSE without matching IF");
 
         // Push merge block for THEN to pick up.
         cfStack.push_back({CFTag::Orig, mergeBlock});
@@ -777,7 +781,11 @@ LogicalResult ForthParser::parseBody(Value &stack) {
         consume();
 
         // Pop the join/merge block.
+        if (cfStack.empty())
+          return emitError("THEN without matching IF");
         auto [tag, joinBlock] = cfStack.pop_back_val();
+        if (tag != CFTag::Orig)
+          return emitError("THEN without matching IF");
 
         // Branch from current block to join.
         builder.create<cf::BranchOp>(loc, joinBlock, ValueRange{stack});
@@ -810,7 +818,11 @@ LogicalResult ForthParser::parseBody(Value &stack) {
 
         auto [s1, flag] = emitPopFlag(loc, stack);
 
+        if (cfStack.empty())
+          return emitError("UNTIL without matching BEGIN");
         auto [tag, loopBlock] = cfStack.pop_back_val();
+        if (tag != CFTag::Dest)
+          return emitError("UNTIL without matching BEGIN");
 
         auto *exitBlock = createStackBlock(parentRegion, loc);
 
@@ -829,7 +841,11 @@ LogicalResult ForthParser::parseBody(Value &stack) {
 
         auto [s1, flag] = emitPopFlag(loc, stack);
 
+        if (cfStack.empty())
+          return emitError("WHILE without matching BEGIN");
         auto [tag, loopBlock] = cfStack.pop_back_val();
+        if (tag != CFTag::Dest)
+          return emitError("WHILE without matching BEGIN");
 
         auto *bodyBlock = createStackBlock(parentRegion, loc);
         auto *exitBlock = createStackBlock(parentRegion, loc);
@@ -851,13 +867,21 @@ LogicalResult ForthParser::parseBody(Value &stack) {
         consume();
 
         // Pop loop header (from WHILE's re-push).
+        if (cfStack.empty())
+          return emitError("REPEAT without matching WHILE");
         auto [destTag, loopBlock] = cfStack.pop_back_val();
+        if (destTag != CFTag::Dest)
+          return emitError("REPEAT without matching WHILE");
 
         // Branch back to loop header.
         builder.create<cf::BranchOp>(loc, loopBlock, ValueRange{stack});
 
         // Pop exit block (from WHILE).
+        if (cfStack.empty())
+          return emitError("REPEAT without matching WHILE");
         auto [origTag, exitBlock] = cfStack.pop_back_val();
+        if (origTag != CFTag::Orig)
+          return emitError("REPEAT without matching WHILE");
 
         // Continue after exit.
         builder.setInsertionPointToStart(exitBlock);
@@ -1007,6 +1031,11 @@ LogicalResult ForthParser::parseBody(Value &stack) {
     }
   }
 
+  if (!cfStack.empty()) {
+    cfStack.clear();
+    return emitError("unclosed control flow (missing THEN, REPEAT, or UNTIL?)");
+  }
+
   return success();
 }
 
diff --git a/test/Translation/Forth/begin-begin-repeat-mismatch-error.forth b/test/Translation/Forth/begin-begin-repeat-mismatch-error.forth
new file mode 100644
index 0000000..d9d7b7d
--- /dev/null
+++ b/test/Translation/Forth/begin-begin-repeat-mismatch-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: REPEAT without matching WHILE
+\! kernel main
+BEGIN BEGIN REPEAT
diff --git a/test/Translation/Forth/begin-then-mismatch-error.forth b/test/Translation/Forth/begin-then-mismatch-error.forth
new file mode 100644
index 0000000..16a1708
--- /dev/null
+++ b/test/Translation/Forth/begin-then-mismatch-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: THEN without matching IF
+\! kernel main
+BEGIN THEN
diff --git a/test/Translation/Forth/else-without-if-error.forth b/test/Translation/Forth/else-without-if-error.forth
new file mode 100644
index 0000000..7179e26
--- /dev/null
+++ b/test/Translation/Forth/else-without-if-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: ELSE without matching IF
+\! kernel main
+ELSE
diff --git a/test/Translation/Forth/if-repeat-mismatch-error.forth b/test/Translation/Forth/if-repeat-mismatch-error.forth
new file mode 100644
index 0000000..70b62c9
--- /dev/null
+++ b/test/Translation/Forth/if-repeat-mismatch-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: REPEAT without matching WHILE
+\! kernel main
+1 IF REPEAT
diff --git a/test/Translation/Forth/if-until-mismatch-error.forth b/test/Translation/Forth/if-until-mismatch-error.forth
new file mode 100644
index 0000000..16f4283
--- /dev/null
+++ b/test/Translation/Forth/if-until-mismatch-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: UNTIL without matching BEGIN
+\! kernel main
+IF UNTIL
diff --git a/test/Translation/Forth/repeat-without-while-error.forth b/test/Translation/Forth/repeat-without-while-error.forth
new file mode 100644
index 0000000..8728fae
--- /dev/null
+++ b/test/Translation/Forth/repeat-without-while-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: REPEAT without matching WHILE
+\! kernel main
+REPEAT
diff --git a/test/Translation/Forth/then-without-if-error.forth b/test/Translation/Forth/then-without-if-error.forth
new file mode 100644
index 0000000..8c8ada6
--- /dev/null
+++ b/test/Translation/Forth/then-without-if-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: THEN without matching IF
+\! kernel main
+THEN
diff --git a/test/Translation/Forth/unclosed-begin-while-error.forth b/test/Translation/Forth/unclosed-begin-while-error.forth
new file mode 100644
index 0000000..1d56b55
--- /dev/null
+++ b/test/Translation/Forth/unclosed-begin-while-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: unclosed control flow (missing THEN, REPEAT, or UNTIL?)
+\! kernel main
+BEGIN 1 WHILE 42
diff --git a/test/Translation/Forth/unclosed-if-error.forth b/test/Translation/Forth/unclosed-if-error.forth
new file mode 100644
index 0000000..7a156d4
--- /dev/null
+++ b/test/Translation/Forth/unclosed-if-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: unclosed control flow (missing THEN, REPEAT, or UNTIL?)
+\! kernel main
+1 IF 42
diff --git a/test/Translation/Forth/until-without-begin-error.forth b/test/Translation/Forth/until-without-begin-error.forth
new file mode 100644
index 0000000..576d10f
--- /dev/null
+++ b/test/Translation/Forth/until-without-begin-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: UNTIL without matching BEGIN
+\! kernel main
+UNTIL
diff --git a/test/Translation/Forth/while-without-begin-error.forth b/test/Translation/Forth/while-without-begin-error.forth
new file mode 100644
index 0000000..e26bac0
--- /dev/null
+++ b/test/Translation/Forth/while-without-begin-error.forth
@@ -0,0 +1,4 @@
+\ RUN: %not %warpforth-translate --forth-to-mlir %s 2>&1 | %FileCheck %s
+\ CHECK: WHILE without matching BEGIN
+\! kernel main
+WHILE