This can also be caused by shorting out the circuit with flashes. If the flash has power on when put on or taken off, and the central sync contact on the flash touches either of the two dedicated pins in the back, it will damage the circuit. In most cheap flashes (and almost all old ones) the full voltage of the flash (over 300 volts) is on the sync contact in the center, and there is a switch in the shutter that causes the circuit to complete. Later flashes used a 3 volt sub circuit to complete the circuit, isolating the camera from the full 300 volts.
If the flash sync contact that has 300 volts touches either of the dedicated pins, then you have 300 volts going into a 6 volt circuit. This usually damages either IC3 (PX3) or both IC3 and IC2 (PX-3 and PX-2). These parts haven't been available for nearly 15 years. Damage to these pins can cause problems like the aperture opening all the way or closing down all the way every shot, or the curtains traveling together every shot (blank shots).