Not really a frame, a drive side dropout sheared all the way through at the junction with the chainstay, Mondonico Futura Leggero (custom geo - I was measured by Antonio Mondonico himself!! His son Mauro was there too.).
I believe that this was the result of an almost-crash several years before the dropout cracked. I was riding in the early morning before work, therefore dark. Descending a hill where I regularly go over 40 mph. There was an unmarked construction trench dug across the road and not completely filled in, so there was a trench about 4 inches wide and an inch and a half deep stretching completely across the road that I hit in excess of 30 mph. No problem for cars, I guess, but it blew both my tires and dented both of my rims. I managed to stop without crashing, fortunately, and made the call of shame. I suppose that this started a small crack in the part of the dropout that goes inside the chainstay tube, or perhaps it was just a stress point that later developed into a crack - at any rate, I never noticed anything with the dropout until it cracked (fortunately less than a mile into my ride, so I was able to nurse it home).
I need to get that fixed some day, the frame is still in my basement.