OOD Mistake Clinic

Module-Specific Mistake Radar

Mistake	Discriminating evidence	Repair
Private field means stable history	Mutate a retained nested element	Specify ownership and element immutability
Same signatures prove substitution	Rejected withdrawal changes state	Shared state-machine contract and public trace
Every large function needs splitting	A coherent pure kernel and its invariant	Compare comprehension and boundary cost
An interface needs two production providers	One real clock plus deterministic testing	Name present benefit and carrying cost
Fewer comments is better	Missing units or failure contract after deletion	Preserve useful API documentation
Renaming cannot break behavior	Reflective client or persisted key	Compatibility inventory and migration
Cleaner extraction preserves behavior	Iterator exhaustion or late failure	Trace effects before and after
Repeated field access proves Feature Envy	An intentional serializer	Identify policy ownership before moving code
Wrapping a primitive guarantees validity	NaN, Boolean amount or mismatched currency	Explicit construction and operation validation

Practice Mistake Checks

Use P02 for aliases, P04 for actual parent contracts, P08 for output order, P10 for published compatibility, P14 for one-provider seams and P19 for failure boundaries. Record a real failure or uncertainty, the hidden assumption, repair and changed-case check. Do not invent a mistake quota. The practice feedback follows an independent attempt.