Mistake Clinic

Module-Specific Mistake Radar

Symptom	Underlying error	Small falsification
Sorted keys but reordered ties	Compared payloads or chose the right merge head on equality	Two equal keys with distinct tags
Search returns any matching duplicate	Wrong output contract	Three equal keys and a boundary query
Search stalls	No strictly shrinking interval	One-element active interval with lo=mid
Random-pivot quicksort is quadratic on ties	Equal keys all sent to one recursive side	All-equal array
Selection called worst-case linear	Expected bound used as a guarantee	Extreme pivot sequence
Deleted neighbor becomes unreachable	Empty and deleted states conflated	Collision chain; delete its first entry
Old map value reappears	Inserted at tombstone before finding existing key	Delete head, update later key, delete it
Low-load map is slow	Deterministic collision family ignored	Multiples of capacity
Heap construction analysis is n log n	Charged root height to all nodes	Height-sum derivation
Merge exhausts memory	Flattened inputs or retained all output internally	Guarded generators and source inspection
External-sort time is halved	Counted merge but omitted run generation	Read/write ledger for every stage
Radix allegedly sublinear	Dropped pass ceiling and input traversal	Fixed-width keys still require one pass

Practice Mistake Checks

For P1–P4, verify stability, multiplicity and model assumptions separately. For P5–P8, write bounds and rank conventions before code. For P9–P12, use operation histories and name the probability space. For P13–P16, price input/output and retained state rather than only the central loop.

Record a real failure, the smallest reproducing input, violated invariant, repair, and fresh successful case. A copied fix without an explanation remains incomplete. Use Practice Solutions only after preserving your attempt.