diff --git a/test/check_ledger.tsv b/test/check_ledger.tsv
index 9ba482f2b..ab3782fc5 100644
--- a/test/check_ledger.tsv
+++ b/test/check_ledger.tsv
@@ -768,7 +768,7 @@ differential	differential	stripe boundary N=2001 agg	15;16;17;18;19	never	-
 differential	differential	stripe boundary N=2001 scan	15;16;17;18;19	never	-
 differential	differential	stripe boundary N=2001 stripes	15;16;17;18;19	never	-
 differential	differential	textbloom C absent	15;16;17;18;19	never	-
-differential	differential	textbloom C eq	15;16;17;18;19	never	-
+differential	differential	textbloom C eq	15;16;17;18;19	2026-10-16	the reader's bloom check skipping every chunk under the C collation (mutation, #1339)
 differential	differential	textbloom absent	15;16;17;18;19	never	-
 differential	differential	textbloom collate-mismatch	15;16;17;18;19	never	-
 differential	differential	textbloom present	15;16;17;18;19	never	-
diff --git a/test/check_ledger_budget.txt b/test/check_ledger_budget.txt
index e9dc0d4b3..a14fe9df7 100644
--- a/test/check_ledger_budget.txt
+++ b/test/check_ledger_budget.txt
@@ -3619,4 +3619,14 @@ suites_not_covered 185
 #     never         3074 +  5 = 3079
 #     dated         1366 +  3 = 1369
 #     3079 + 1369 == 4448
-checks_never_observed_red 3079
+# #1339, differential's text-bloom C arm probes a value that is there (jdatcmd):
+# 3079 -> 3078. "textbloom C eq" probed tc = 'c123', which is in no row, so both
+# sides counted 0; it now probes row 7's value, and the reader's bloom check
+# skipping every chunk under the C collation dates it. No row is added, removed or
+# renamed.
+#
+#     ledger rows   4448 + 0 = 4448
+#     never         3079 - 1 = 3078
+#     dated         1369 + 1 = 1370
+#     3078 + 1370 == 4448
+checks_never_observed_red 3078
diff --git a/test/differential.sh b/test/differential.sh
index 2c3e613d7..c55e50828 100755
--- a/test/differential.sh
+++ b/test/differential.sh
@@ -344,7 +344,10 @@ load_pair "SELECT g, 'k' || ((g*2654435761)%50000), 'c' || ((g*40503)%50000)
 diff_query "textbloom present"  "SELECT id FROM %T WHERE tk = 'k' || ((7*2654435761)%50000)"
 diff_query "textbloom absent"   "SELECT count(*) FROM %T WHERE tk = 'zzzzzzzz'"
 diff_query "textbloom C absent" "SELECT count(*) FROM %T WHERE tc = 'zzzzzzzz'"
-diff_query "textbloom C eq"     "SELECT count(*) FROM %T WHERE tc = 'c123'"
+# A PRESENT value (#1339). 'c123' is in no row (tc is 16,000 distinct values of 50,000),
+# so both sides counted 0 and a bloom wrongly skipping the chunk could not show. Row 7's
+# value is present exactly once, the way "textbloom present" probes tk.
+diff_query "textbloom C eq"     "SELECT count(*) FROM %T WHERE tc = 'c' || ((7*40503)%50000)"
 # mismatched explicit COLLATE must still return correct results (not pushed)
 diff_query "textbloom collate-mismatch" "SELECT count(*) FROM %T WHERE tk = 'k100' COLLATE \"C\""
 
diff --git a/test/pytest/test_differential.py b/test/pytest/test_differential.py
index 4c190dc76..e56088e64 100644
--- a/test/pytest/test_differential.py
+++ b/test/pytest/test_differential.py
@@ -288,7 +288,15 @@ def test_sum_and_avg_agree_for_every_numeric_type(matrix, col, expect):
         expect.row_set(c, h, f"{col} sum/avg")
         return
     cs, hs = c[0], h[0]
-    expect.num(len(cs), len(hs), f"{col} sum/avg returns the same shape")
+    # TWO NUMBERS ON BOTH SIDES, not the same length (#1339). len() of a two-column row
+    # is 2 whatever the data, so a columnar sum of NULL passed that arm and then raised
+    # an unnamed TypeError in the tolerance below.
+    numbers = all(isinstance(v, (int, float)) for v in (*cs, *hs))
+    expect.text("two numbers each" if numbers and len(cs) == len(hs) == 2
+                else f"columnar {cs!r} heap {hs!r}",
+                "two numbers each", f"{col} sum/avg returns two numbers on both sides")
+    if not numbers:
+        return
     for got, want, what in zip(cs, hs, ("sum", "avg")):
         # THE MEASUREMENT BELONGS IN THE COMPARED VALUE, NOT IN THE NAME. This put
         # `got`, `want` and `rel` into the check NAME, which does carry them to the
@@ -839,7 +847,9 @@ def test_text_bloom_equality_agrees_including_a_mismatched_collation(pgc_conn, e
             ("textbloom present",           "SELECT id FROM %T WHERE tk = 'k' || ((7*2654435761)%50000)"),
             ("textbloom absent",            "SELECT count(*) FROM %T WHERE tk = 'zzzzzzzz'"),
             ("textbloom C absent",          "SELECT count(*) FROM %T WHERE tc = 'zzzzzzzz'"),
-            ("textbloom C eq",              "SELECT count(*) FROM %T WHERE tc = 'c123'"),
+            # A PRESENT value (#1339): 'c123' is in no row, so both sides counted 0
+            # and a wrongly skipped chunk could not show. Row 7's value is present once.
+            ("textbloom C eq",              "SELECT count(*) FROM %T WHERE tc = 'c' || ((7*40503)%50000)"),
             ("textbloom collate-mismatch",
              f"SELECT count(*) FROM %T WHERE tk = '{present}' COLLATE \"C\"")):
         c, h = p.both(sql)