Single event upset (SEU)
A bit that changes value because a charged particle happened to pass through the transistor — a fault with no bug and no code to blame.
An ionising particle leaves a trail of charge in the silicon, and if that is enough to flip a memory cell or flip-flop you have a soft error: the circuit is undamaged, the data is wrong. In space it is routine. At ground level, secondary neutrons from cosmic rays account for most of them, and the rate climbs with altitude and with every transistor shrink.
The most quoted case is the 2003 municipal election in Schaerbeek, Belgium, where one candidate received exactly 4,096 extra votes — a single bit. Countermeasures include ECC memory, scrubbing that reads and corrects in the background, lockstep cores checking each other, triple modular redundancy and, as a last resort, a watchdog to restart whatever hung anyway.