It has happened to me, with great luck and realizing the problem sooner than later, I have the piston with the small brass pin that embedded (rather loose term, more like smeared) itself in the side of the piston. The ends of the ring were a bit chewed up but so little you could still use them again if you wanted, the wall of the cylinder and piston had some good vertical scratches.
My guess is it fell out or shorn off at the perfect moment and got tossed somehow or another to a spot where it was immediately burnished into the piston.
Could have even been s**tty soft China brass, that could be soft enough not to muck up things, mind you dimples form from crushing, crushing means a squish clearance small enough to actually be capable of crushing such a small object.
If all the motors that lost a pin and also self destructed then the dimples may also be large pieces of shrapnel from said self destruction phase, not necessarily just the pin.