## What
A `docs/solutions` note recording three diagnoses I reversed **in one
session**, all wrong the same way. Docs only.
## The three
| observed | my story | what it was |
|---|---|---|
| `planning-browser-e2e` fails at width **769**, passes at **768** |
layout regression at the tablet breakpoint, from FN-8606 | a **race** —
5 passes in 6 runs; on every pass the control sits inside the viewport
at 769 (`right: 753 ≤ 769`) |
| a model-routes test fails **3 of 3** locally | red on main; quarantine
candidate | **green on CI**; a sandbox interaction. The fixture is
configured offline, so a sandbox should not have changed the outcome —
the tell was there from run one |
| one approach could not cover a resolver | the site is **unpinnable** |
a *different shape* covered it — a helper that **resolves** rather than
one that **receives** |
Each was plausible, mechanistic, and consistent with the evidence I had.
That is what made each dangerous: **a diagnosis that explains your one
data point feels finished.**
Each survived exactly until a second measurement **of a different kind**
— another environment, more samples, an instrumented probe. Re-running
the same command is not a second measurement.
## The reusable part
| observation | tempting story | check first |
|---|---|---|
| fails at boundary X, passes at X−1 | structural bug at the boundary |
run it 5 more times — boundaries are where races surface |
| **consistent** locally, green on CI | main is broken | the
environment; consistency is not universality |
| **intermittent** locally, consistent on CI | flaky test | a race the
slower runner loses every time |
| one approach failed | the site cannot be done | whether a different
*shape* of the approach works |
## Why it matters beyond debugging hygiene
Two of the three would have caused real damage if acted on:
- Quarantining the model-routes test — the action the standing rule
seems to license on "observed failing" — would have **deleted coverage
that is green everywhere else**. The rule's *"without a corresponding
real bug"* clause is load-bearing, and a local observation does not
satisfy it.
- "Unpinnable" hardened a single failed approach into a property of the
site. Left standing, it becomes a permanent excuse not to look — the
same failure I corrected in an inherited note earlier today, which had
recorded four resolvers as unmeasurable for environment reasons that did
not hold here.
Hence the last rule: **record cautions as environment-scoped, not as
properties of the code.** Say where you measured.
```
lint clean; fnxc-future-dates: none added (exit code checked before piping)
```
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