Here's the craziest of them all: You forgot to wire up the Vdd pin on a CMOS chip, but the circuit works just fine! That's because it is being powered by one of its logic inputs (via the input-protection diodes from the input to the Vdd line in the chip). You might get away with this for a long time, but suddenly the circuit reaches a state where all the logic inputs to the chip are simultaneously LOW; the chip loses its power and forgets its state. Of course, this is a bad situation anyway, since the output stage isn't adequately powered and can't source much current. The trouble is that this situation may produce symptoms only occasionally, and it can have you running around in circles until you figure out what's going on.