I was cutting along just fine when it began to rattle. Shut down, nothing noticeable, fired up and finished. Wouldn't start again. Tore it down and found the electrode had separated of the plug and beat the bejabbers out of the top of the piston including peening over the ring lands. Cost me a new piston rod assy. Still haven't figured out how it started after I shut it down the first time.
1951 The old man's Caterpillar 22 (small gas tractor about D2 size built back in the 30s I think). Had a full length cover over the plugs. Cranked it one day, big bang and it froze up. Couldn't pull it an inch with the crank. Plug had blown out and apparently blew all the oil out of that cylinder. We managed to get it to rotate using a pry bar - ran just fine for years after but that #1 plug had to be retightened regularly.
Harry K