Refactoring Mistake Clinic

Module-Specific Mistake Radar

Signal	Why it fails	Repair
Green tests called a proof	Untested inputs and observations remain	State domain, oracle and transformation argument
Starter passing called completion	It already passes before editing	Review intermediate diffs and process evidence
Loop replaced during extraction	Traversal, holes or getters may differ	Preserve traversal first; test a second move separately
New validation hidden in wrapper	Accepted inputs and error order change	Declare an intentional repair/migration
Old/new APIs use separate stores	Mixed clients disagree	One authority or an explicit synchronization design
Timeout triggers fallback write	Original effect may have committed	Preserve uncertainty and define reconciliation
Alias removed after local search	External/delayed callers remain	Inventory supported consumers and retention policy

Practice Mistake Checks

For P02 and P06, preserve rounding location and the deadline equality case. For P07 and P11, record evaluation count and order. For P09, use two invalid tokens at once. For P16, interleave APIs on one instance. For P17–P19, separate routing recovery from persisted state and external effects.

When a check fails, retain the smallest witness, isolate the last change and state whether it is a regression, brittle test, fixture fault or intended repair. Do not manufacture a quota of mistakes. Recheck a changed case after correction.