Oh, it happens often enough, in all sorts of different equipment. I've seen something similar happen a couple of times in VCRs, but in that case there is normally a horrible grinding noise as the mechanism becomes irretrievably wedged solid. I recently stripped down a ford DOHC engine sourced from a running vehicle, only to find a fairly large screw jammed solidly in the timing chain, which would explain the noise Screws unscrewing themselves is always a possiblilty wherever you have vibration, and it's almost impossible to eliminate completely. At least without a welder, which makes maintainance difficult...

pca