In digital circuits, a runt pulse is a narrow pulse that, due to non-zero rise and fall times of the signal, does not reach a valid high or low level. A runt pulse may occur when switching between asynchronous clocks; or as the result of a race condition in which a signal takes two separate paths through a circuit, which may have different delays, and is then recombined to form a glitch; or when the output of a flip-flop becomes metastable.