dabac

Haven't seen may failures where there's an obvious link from a previous impact damage. What I've got is mostly usage related.
There's one regularly ridden bike by my train station which has had the top tube bent down with a good 4-6" that is still holding together. Don't know the mileage though, it might just go a block or two.

Lessee....
- I've had one seat tube fail mid-shaft while JRA. Turned out there was a foam plug down there, probably to prevent water from getting into the BB. As a result, the tube rusted out at that point. The bike had signalled that there was something wrong by the extra flex in the frame causing the rear wheel to rub at every pedal stroke for a few minutes before complete separation occurred.
- My brother had a threaded steerer tube come apart between top race and locknut, but by leaning on the bars he maintained sufficient control to bring the bike to a safe stop.
- a co-worker had a dropout separate from the chainstay on a high-mileage bike. He also managed a safe stop. Stick welded it back together and has logged plenty of miles since.
- a friend had an aluminium steerer tube fail inside the head tube. Would have turned nasty if he had pulled a wheelie. Can't think of what, apart from some material flaw or bad scratches from assembly to initiate such a failure.