Extends the doc from #3255/#3273 with the failure that cost the most in
a single session: **one stale install produced five wrong reports on one
issue** (#3264).
## What happened
A `node_modules` that had drifted from the lockfile — `jsdom@29.0.1`
installed, `29.1.1` pinned — generated failures that existed on no CI
machine and no other checkout. They were not subtle: deterministic,
reproducible on demand, with plausible stack traces and real-looking
assertion diffs.
Each round of triage got **more precise about the wrong data**:
| round | claim | why it was wrong |
| --- | --- | --- |
| 1 | "4 deterministic failures" | measured in a 4-file batch, called it
isolation |
| 2 | "3 deterministic, 2 order-dependent" | isolated correctly, but a
race is not deterministic |
| 3 | "TaskCard is broken" | stale jsdom; the CSS assertion was correct
|
| 4 | "no contamination" | true of four app files; published unqualified
|
| 5 | "quarantine these two" | never read the failure text — both were
timeouts |
The through-line is not carelessness about the code. **The environment
was never treated as part of the claim**, so no amount of care about the
analysis could recover it.
## The checks, in the order they cost the most
```bash
pnpm install --frozen-lockfile # node_modules is not evidence until it matches the lockfile
<run the file ALONE, 3+ times> # isolation and repetition answer different questions
<read the failure TEXT> # a timeout and an assertion failure need opposite responses
uptime # a loaded box manufactures timeouts that mean nothing
```
## Why the load check earned its place
Two tests "failing" in a full-suite run were `Test timed out in 15000ms`
on a box at **load average 9.7 with 84 users**. Under AGENTS.md's
quarantine-on-sight rule that reads as a flake to quarantine — and the
ledger's **14-day deletion ratchet would have made the lost coverage
permanent**.
The rule presumes the failure is a property of the test, not of the
machine. A wall-clock budget crossed under local contention is evidence
about the hardware. I was one comment away from deleting healthy
coverage on that basis.
## The tell
A finding is environment-derived when it is **local, recent, and
unshared**: nobody else has reported it, CI is green, and it appeared
without a commit that could explain it. Any two of those should stop a
report before it is written. All three applied here, and the report went
out anyway — five times.
## Verification
Docs only; no code paths change. `fnxc-future-dates`,
`lifecycle-columns`, `quarantine-ledger` exit 0. No changeset — internal
docs are excluded.
Context: the one finding in #3264 that survived all five rounds is #3286
(merged), and it survived because it was verified by **reverting the
product change** rather than by trusting a red — 3/3/2 failures without
the fix, 27/27 across four runs with it.
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