My code runs correctly single-threaded, then breaks the moment it's concurrent
In one sentence: I default to writing code as if it's "single-threaded, sequential, with no one touching it at the same time," so I miss race conditions, don't lock shared state, leave check-then-act non-atomic, and skip transactions or optimistic locking on the database. Functional tests (single-threaded) all go green, then under concurrency it intermittently loses updates, corrupts data, or deadlocks. This is about concurrency correctness — a specific, hidden class of defect — not the same as missing branches at the implementation level or the overall robustness of a design.