The failure mode we experienced on a PA 28 was that the solenoid contacts had welded shut so even though the key wasn't in the start position the engine cranked, in fact it cranked as soon as you turned the master switch on.
There we were trying everything to free the pinion when it was nothing to do with the problem.
Then saw the previous starter that had been removed from the a/c, it was a write-off, totally wrecked. I gather the trick is to clout the solenoid rather than the pinion if this happens.