diff --git a/context/reconstruction-gotchas.md b/context/reconstruction-gotchas.md
index 2309835..b3d9793 100644
--- a/context/reconstruction-gotchas.md
+++ b/context/reconstruction-gotchas.md
@@ -1106,3 +1106,25 @@ THIS NEVER RUNS *** banner naming the live site (missile.cpp now does).  (3) A d
 FIXED is a claim, not evidence -- a fix without a FIELD receipt ([incoming], [seqloop], etc.)
 is T3 at best.  (4) The fix's own bench must fail first: the first #83 bench (solo) produced
 zero receipts because solo seats never fill -- proving the rig, then the fix, in that order.
+
+## 34. CHANGE DETECTION AGAINST A SHARED MULTI-WRITER SCRATCH: previousModeMask (#155, 2026-08-14)
+The engine ModeManager keeps ONE previousModeMask cell, overwritten by EVERY
+Add/Remove/ReplaceModeMask call.  Any consumer computing `changed = current ^
+manager->GetPreviousModeMask()` only sees the delta of the LAST write -- fine
+while exactly one writer touches the mode per frame, silently lossy the moment
+a second writer appears.  The authentic panic-arm chatter (the no-hysteresis
+eject evaluator flapping ModeEject per frame) made the gauge renderer's page
+XOR permanently blind to SetPresetMode's page bits: MFD page EXITS died while
+the PANIC lamp flickered (entries survived via !alreadyActivatedFlag; the
+always-active radar/score/coolant never consult the XOR -- the field's exact
+alive/dead split).  The renderer ALREADY OWNED a correctly-maintained
+previousModeMask member; reading the manager's scratch instead looks like
+lineage drift.
+**Rules:** (1) a change-detector must own its OWN baseline snapshot, sampled
+and advanced at ITS consumption rate -- never a shared scratch another writer
+can stomp between samples.  (2) When an authentic high-frequency toggler
+exists (chatter, flap, blink), audit every `GetPreviousModeMask`-shaped read
+it can starve (LAMP.cpp's read survives because lamp flicker IS the intended
+output).  (3) The live-human receipt loop cracked what two headless rigs
+could not: the injection path was clean, so the defect HAD to be presentation
+-- receipts proving delivery while the screen disagrees convict the renderer.