In my 35 years of messing with this stuff, I have seen numerous failures and the only calamitous ones were at the head tube. Somewhere I have a picture of a guy in my Carolina Cup walking back... with the front head tube fork and wheel in one hand and the rest in the other... As Brian said, down tubes alone and bb breaks can often be ridden home despite the sudden "squishy" feeling!
Dale
Dale Brown
Greensboro, North Carolina