Friend of mine had something similar happen many moons ago. I was holding the plane for him while he ran it up, and POP! The stem blew right out of the plug! Startled both of us, but it went straight up and didn't hit anybody. At first we thought it threw the rod or broke the crank or something (no compression)...
Sure is nice to just be able to put a new plug in 'er and fire it back up, with no harm done