I have posted earlier in this thread, that I had one "pop" on me, and still have the scar on the back of my finger to remind me

It was back in the mid 70's and an old "mentor" (you might call him ) gave me a couple of his control line engines from the 50's...
The engine was quite stiff and gummy from castor residue... I put some fuel in it to loosen it up, and as I was holding it in my left hand, and flipping the prop w/ my right (as I have done many many times before ) it popped on me and cracked me on the back of the index and middle fingers...and surprised the heck out of me!

Edit: obviously, that does not fit the description of the thread title...but something did happen, without any battery attached to the plug.