diff --git a/persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md b/persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md
index eece24e..f58d61a 100644
--- a/persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md
+++ b/persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md
@@ -48,3 +48,22 @@ rules (position, alphabetical, count) disagree.
 Whether it becomes `scripts/` is an open question for the operator — this repo
 has now been bitten by vacuous falsifiers three times, and prose in a memory
 file is not an instrument.
+
+## A third way an instrument goes blind: `nth-child` vs `nth-of-type`
+
+_Added 2026-09-23, credited to design-dev, who hit it in his R2 order check._
+
+His layout check has a positive control — one tile given `order:-1` that the
+check must catch. **The control went blind when group headers became grid
+children**: `nth-child(5)` started landing on a header instead of the fifth
+tile, so the control stopped controlling and the check kept reporting clean.
+
+Same class as this file's other two, and the reason it belongs here: **a control
+that no longer controls reads exactly like a passing test.** Nothing in the
+output distinguishes "detected nothing because there was nothing" from
+"detected nothing because I am aimed at the wrong element".
+
+**The rule worth having written down:** use `nth-of-type` over `nth-child` for
+any assertion that means *the Nth TILE* rather than *the Nth child element*.
+The two agree right up until somebody adds a sibling of a different kind — and
+adding a sibling is what a redesign is.