A pretty common occurrence with US grounded plugs... the hollow ones break off all the time, which are generally found on molded plugs.
Many of those were probably people wanting to plug it into a non-ground outlet, and had no adapter handy...
Back in the late 70's at college, my dorm had the crowfoot configuration outlets (the Aussie style pattern) in the laundry room, and whoever installed new washing machines took pliers and crushed the ground pin and twisted the blades on the NEMA plugs to make it fit into the crowfoot outlet...
