Reference for the correctness sub-case — the core of the code-review skill.
Reference for the correctness sub-case — the core of the code-review skill. The performance and security sub-cases have their own files alongside this one.
Overlapping diagnostic lenses, not a classification scheme and not a quota. Each entry says how you detect it, because a class name alone changes nothing about what a reviewer looks at.
Lenses 1 and 2 are the ones strong reviewers — human and machine — miss most often, and the only two that warrant an explicit forced probe rather than a read-through. Both are failures of looking, not of judgement: the code reads sensibly at each participant, and the defect exists only in the relationship between them.
Follow a proposed value through validation, normalisation, negotiation or commit. Identify downstream state still derived from the proposal when the authority can legitimately return something different. Check that reconciliation happens before the first consequential use.
A requested value is not an applied value. Anywhere a system can say "I heard you, and here is what I actually did", the reply is the authority — and the request is not.
Probe: construct an execution where the effective answer differs from the requested one, then ask which stored state, which UI, which later call and which log line still carry the request.
Trace how an operation's result finds its originating entity. Establish the correlation key's uniqueness, stability and lifetime under overlapping operations, reordering, removal and reuse. A label, an index, a position or arrival order is suspicious exactly when one of those can fail.
Probe: run two operations concurrently and complete them out of order. Then remove one mid-flight and reuse its slot.
Mark every value captured before a yield, await, callback, timer or lock release. Determine what can change before it is used, and whether the operation still targets the intended entity and version. Check what the code actually does with a stale result — ignore, apply, or apply silently.
Compare every reachable construction, replacement, restoration, reset, failure and termination path. Look for derived fields or cached decisions correctly re-established on one path and wrongly retained on another. Two paths that should end in equivalent state are an agreement like any other.
Split multi-effect operations at each failure and cancellation point. Is partial state permitted, recoverable and accurately reported? Look for success reported before the required effects are durable.
Follow retries, duplicate delivery, repeated callbacks and re-entry into effects. Compare the actual delivery guarantee against the required effect count — especially where an effect costs money, sends a message or mutates a shared total. Inspect deduplication scope, lifetime, and behaviour after partial success.
Trace independently produced pieces through merge, reduction, ordering and dispatch. Compare the real overwrite and selection rules against the intended authority or priority — including transformations applied after the merge that quietly re-order or re-key it.
Follow counts, lengths, offsets, capacities and totals through every transformation. Check empty and boundary cases, overflow, rounding, and whether measurement and consumption use the same unit and representation.
Compare logical item boundaries against actual read, write, iterator and callback boundaries. Test split items, combined items, partial writes and early termination. Inspect buffering, flush and finalisation — especially the last item.
Compare the values a producer can emit against the distinctions a consumer relies on. Trace round-trips and derived outputs for distinctions that disappear. This is the highest-frequency class in every corpus examined - planted and natural - so give it more than one pass.
Three sub-shapes carry most of it:
states that collapse into each other with alarming ease. A failed read returning an empty result, a never-started resource reported as broken, a guard that excludes null but not undefined, an explicit null skipped as though the field were missing, a string "false" landing in a boolean, a sentinel like -1 standing in for "no answer" and then being counted. Probe: for each value that can be missing, enumerate which of the six it can actually be, and check the consumer distinguishes the ones that matter.
emitting a field, and consumers degrade silently rather than failing. Probe: diff the field set a producer actually selects against every field its consumers read, including nested projections and the fields a renderer touches. Also check for a field read under a name the producer never emits.
still-loading state persisted or compared as though it were the settled value. Probe: anywhere a value has a "not ready yet" state, find who reads it without checking.
Then the ordinary axes: signed versus unsigned, canonical forms, precision, encoding, equality semantics.
Tell: a cast, an any, a non-null assertion or a suppressed warning at a boundary is where contracts go to die - the annotation exists precisely because the two sides disagreed and someone silenced the compiler rather than reconciling them. Treat every one of them on a boundary as a candidate.
Trace who may mutate, release, cancel and complete an operation or resource. Inspect exceptional exits and competing terminal paths for premature release, missing completion, deadlock, or use after ownership changed hands.
Enumerate the meaningful states and inputs for consequential predicates and dispatch tables. Compare branches against supported expectations: overlapping conditions, inverted tests, missing cases, and what the fallback actually does. Check the default a framework or language supplies when the code specifies nothing - an unstated default is still a decision.
Include reachability, not just correctness. Ask of each consequential branch whether any input can reach it, and of each scheduled or registered thing whether anything actually starts it. A predicate that can never be true, a handler never wired up, a writer whose output can never reach disk, and a job whose scheduler is never started are all defects that read as perfectly correct code. They are common in the wild and almost absent from planted corpora, so no checklist trained on planted bugs will prompt you to look.
Follow what happens when each step fails, and ask what would make the failure visible. Look for checks that cannot fail, oracles that measure something other than the thing they claim to, exceptions swallowed into a success path, gates that skip their subject, and effects whose absence nothing would detect. A step that always passes is not a passing step.
Probe: for each guarantee the system claims, name the observation that would break if it stopped holding. If there is none, the guarantee is decorative.
This lens exists because of a gap in the evidence, and the gap is worth stating. A planted defect is detectable by construction - somebody planted it, so somebody can find it. Silent failure is therefore systematically absent from planted corpora and heavily represented in real fix histories, where "and nothing noticed" is a recurring phrase. Do not let a benchmark-shaped checklist talk you out of looking here.
probes — a read-through reliably returns nothing here, which is exactly the failure mode.
of the system makes it so.