From time to time I've seen flathead V8s with nasty holes in the sump, block or both where a conrod has let go. I've also seen Chevrolet 216s with the same illness.
I remember going to look at a Morris Minor once which the seller said had a noisy engine. I could grab the crankshaft pulley and move it all around; it had a broken crankshaft! I didn't bother trying to start it.
My only direct experience with this phenomenon was a Briggs 11hp motor which suddenly went thunk. I was wondering whether it has actually happened to anyone while driving and if not too painful would you care to share your story.