Engineering
The check whose failure looks like success
The defect class that has produced more bugs in this codebase than any other, and the one fix that works every time.
The shape
A function answers a question, and its empty case, its error case and its genuine negative all come back looking identical. A count that could not be read returns zero. A scan that matched nothing returns clean. A guard that never ran returns green. Nothing is lying, and every caller draws the wrong conclusion.
The fix
Return what was examined alongside the verdict, and let the type carry the uncertainty so a caller cannot drop it. A rank the board could not answer becomes a dash that says why, not a confident #0. A guard prints the files it read before it says none of them failed.