Embedded OS for a consumer product; the units were freezing (very occasionally) in the field. Usually the units were in store kiosks (= disappointed and unimpr

Embedded OS for a consumer product; the units were freezing (very occasionally) in the field. Usually the units were in store kiosks (= disappointed and unimpressed could-have-been-customers). Turned out to be a race condition in an interrupt handler, where the OS would say "wake me up when something interesting happens" but something else would sneak in, clobber the wake-up trigger, which meant "... never." Two weeks to find it, fixed by swapping two instructions in some assembly glue. The harder a bug is to find, the simpler the fix is.