Defect taxonomy scan

In the reading room	In the review
she reads the request form first: what is the referring doctor asking?	intent first (step 1)
she sweeps the film one region at a time, in the same order every time	one class per pass, from a fixed list (steps 2–3)
she looks twice at the review areas that hide findings: behind the heart, the lung tops	the quiet classes: empty, absent, the error path (steps 4–8)
her report names the region and what she saw there	a finding names its class and its trigger (step 9)
the referral says "fall, rib pain"; she finds the broken rib and still sweeps the rest, where a small nodule sits	keep going after a find (step 11)
a shadow a second view shows to be an overlapping vessel is left out of the report	drop what a test clears (step 12)

In real terms: a diff hides a few classes of bug; asking each class's question of every changed line, with the input that triggers it, finds the ones a single read skims past.

Breaks down when: a film is only looked at, but code can be run. Each pass ends in an executed trigger, not a judgment of shadows, and the MR's own tests are like a film taken from one angle only (step 10).

Contrasting case: a one-line typo fix in a comment has no list, state, error path or resource; a scan has nothing to sweep, and one read is enough.

4.1 ExamHall EXH-3060: five passes on "close the exam"

EXH-3060: ExamHall's "close the exam" MR passed one careful read, and its test, three open attempts with a draft each, was green. On exam day closing ECON-201 failed, and every later try hung. The store's type promises each open attempt a draft; a candidate who opened the paper and never typed has draft: null. Run the passes on the code as merged:

// src/exams/close-exam.ts (as merged)
import type { ExamDeps } from '../infra/types';

export const CLOSE_DEFAULTS = { notify: true };

export async function closeExam(deps: ExamDeps, examId: string, opts: { notify?: boolean } = {}) {
  const o = Object.assign(CLOSE_DEFAULTS, opts);
  await deps.lock.acquire(examId);
  const open = await deps.store.openAttempts(examId);
  for (const a of open) await deps.store.submit(a.id, a.draft.answers);
  if (o.notify) await deps.mail.resultsPending(examId);
  deps.lock.release(examId);
  return { closed: open.length };
}

4.1 as merged MR's test (3 open attempts, a draft each): {"closed":3} · emails 1 · green
4.1 as merged boundaries · 0 open attempts: {"closed":0} · lock held 0 · nothing to find
4.1 as merged absent values · a candidate who never typed (draft null): fails TypeError (Cannot read properties of null (reading 'answers')) · submitted 1 of 3 · red
4.1 as merged state · close e3 with { notify: false }, then close e4 with no options: emails for e4 0 · red
4.1 as merged errors · the store refuses a submit: fails Error · emails 0 · the failure reaches the caller: nothing to find
4.1 as merged resources · close e2 a second time: pending · lock held on e2 · red
4.1 MR !3360 MR's test (3 open attempts, a draft each): {"closed":3} · emails 1 · green
4.1 MR !3360 boundaries · 0 open attempts: {"closed":0} · lock held 0 · nothing to find
4.1 MR !3360 absent values · a candidate who never typed (draft null): {"closed":3} · submitted 3 of 3 · green
4.1 MR !3360 state · close e3 with { notify: false }, then close e4 with no options: emails for e4 1 · green
4.1 MR !3360 errors · the store refuses a submit: fails Error · emails 0 · the failure reaches the caller: nothing to find
4.1 MR !3360 resources · close e2 a second time: fulfilled · lock held on nothing · green
4.1 classes red on the MR: absent, state, resources · on MR !3360: none

Two passes find nothing; three find a defect each, and the resources defect shows only once the absent value has thrown (steps 5, 6, 8). MR !3360 spreads the defaults into a new object, reads a.draft?.answers ?? {} and releases the lock in finally: every trigger is then green.

4.2 Proof domain, chat: a suspicion a test clears

A chat MR adds lastActivity for the thread list's "last message" time. The boundaries pass suspects an empty thread:

// src/messages/last-activity.ts (after the MR)
import type { Message } from '../infra/types';

export function lastActivity(messages: Message[]): number | null {
  const last = messages.at(-1);
  return last?.sentAt ?? null;
}

4.2 suspicion (boundaries): an empty thread throws on .sentAt · trigger: lastActivity([]) -> null · green, so not posted · two messages -> 250

The trigger runs and the test passes, so the suspicion is not posted (step 12); the pass moves on.