schrödinbug
/shroh'din-buhg/ (MIT, from the Schrödinger's Cat thought-experiment in quantum physics) A design or implementation bug that doesn't manifest until someone reading the source code or using the program in an unusual way notices that it never should have worked, at which point it stops working until fixed.
Though (like bit rot) this sounds impossible, it happens; some programs have harboured schrödinbugs for years.