I like what one bullet maker asked (I don't remember which one)
"Exactly at what point in the animals death did the bullet fail?"
The only bullet failures that I've seen are some of the Ballistic Tips comming apart on impact, and not reaching the vitals, and I've had two 7MM RM's, that judging from the exit wounds, they did not expand at all!
In all my deer hunting years, I've only recovered 2 bullets, all my others were pass-throughs. The one I really remember is a 165 gr. Nosler Partitioner in .308 Win that lodged between the shoulder and the hide. It was perfectly mushroomed at least twice the original diameter!
I think that most of the bullet "failures" are a result of a poor choice on the shooters part with the wrong bullet/caliber for the situation!