diff --git a/guides/method-rules.md b/guides/method-rules.md
index beccb84..8751ae2 100644
--- a/guides/method-rules.md
+++ b/guides/method-rules.md
@@ -211,3 +211,37 @@ separate "no success" from "empty candidate list" — a hook on the callee separ
 Corollary, from the same session: **search the notes before the binary.** The dominant RNG consumer
 was already identified in `strategic-turn-internals.md` months of lanes earlier. What was missing was
 never the identification — it was the connection.
+
+## 19. The instrument can perturb the thing it measures — check, do not assume
+
+Lane H bisected an autosave that differed by 4 bytes from a byte-identical input, across
+configurations, and found the cause: **a MinHook detour on one function changed the game's
+behaviour.** The patched 5 bytes land on a clean prologue boundary with no branch target inside,
+and the *suppressed* draw belongs to a function that runs earlier in the turn than the hooked one.
+The mechanism is still undetermined.
+
+Two things follow. First, the ledger built with a different hook set was checked and came out
+**behaviour-neutral** — its numbers stand. Second, that check had never been run before; it came
+out right by luck, not by design.
+
+So: **before trusting a measurement, take the same measurement with the instrument removed.**
+`hooks=off` in two fresh processes is cheap and it is the control. A hook that changes the autosave
+has invalidated every number taken with it installed, and you will not notice from inside the run.
+
+Corollary for the oracle: re-run it after any change to the hook set, not only after engine changes.
+
+## 20. A count cannot separate "did not fire" from "fired and found nothing"
+
+`CreateRaidEncounter` cost 0 words on every measured turn, which was read as "the roll never
+succeeds (~11%, unremarkable over three turns)". An entry probe showed it is **entered 2x on one
+turn and 1x on the next, drawing 0 every time** — the rolls succeed at about the predicted rate and
+the *candidate list is empty*. Also, three quiet turns at that rate is 1-in-720, not unremarkable:
+when a prior says "unremarkable", compute it.
+
+Same shape elsewhere in the same lane: four tail callees were "never observed firing" and turned out
+to be **entered every single turn and gated inside** — a much stronger negative than reachability.
+And a residual measured at "2 words per turn" on two saves is **one word per player that passes a
+gate**, where exactly one player passes on both corpus saves. A constant fitted to two observations
+is not a constant.
+
+Instrument the **entry**, not just the cost.