Had a rear dropout snap once, at the point where it entered the main frame. Old steel frame, very lightweight, bought off ebay. No markings or decals - I think it had once been a team bike, specially made. Lovely thing it was. But when I bought it it had solid rubber tyres, obviously for awhile (they were very worn) and the ride was horrendous, as you might imagine. My theory was that the lack of pneumatic 'buffering' had overloaded that joint with impact-stress, leading to the failure. I kept it for a few years, hoping to find someone to fix it for me, but I never did and in the end I dumped it.