It's definitely rare. Was the distributor shaft seized to the distributor housing? Don't forget the oil pump is driven by the distributor shaft too, so use a long slot tip screwdriver to make sure it spins freely. If the oil pump was sucking up/ingesting foreign material it would have put a tremendous load on the shaft and the gears. However, the distributor shaft gear has a sheer pin, so even if the pump or the distributor seized it should have sheered off and saved the cam gear. Perhaps there was a quality issue with that cam and the gear just fell apart? Tim