I had something like this happen in the shop this week - next time look at the plug. I found the small shoulder at the base of the threads where the o ring sits, it seemed to allow the plug to thread in too far and the plug spun in the drain opening. A standard brass threaded plug (no O-ring) snugged up tight to the bottom of the drain opening. We've seen cases where the plug was overtightened and the threads were damaged, I suspect that might have been the case.
