I once had a co-worker whose side project was a little wireless weather station transmitter (back in the late 90s). He was getting reports of the batteries drai

I once had a co-worker whose side project was a little wireless weather station transmitter (back in the late 90s). He was getting reports of the batteries draining in a matter o days, when in testing (and in theory) they should last months. After a lot of head scratching he discovered that the bit-change operation on the I/O port of the chip he was using was doing a read-modify-write routine. One of the pins on the I/O port (intended to be "write-only") was attached to a status LED. In bright sunlight, the status LED was generating enough power to read high on the I/O port, so any unrelated bit flips on the port could cause the LED to turn and remain on in bright sunlight.