UA,

Sometimes the propnut will come off if the engine backfires, or spits. (a little to rich) This is more true with engines that are new, or newer. no big deal, run in the engine awhile without the spinner, and when it is broken in and you are more used to it, put the spinner back on. I've only had one come off in the air, a Veco .19 (oldie), but had a lot of my engines spit off the propnut when new. More than likely it didn't hurt the engine if you stopped it quick.

Jetts