I started a very similar thread a few years ago, it is a topic that interests me. After having been around this forum for a few years I think a broken knife is very rare. I can remember a thread about one, a RAT knife that someone broke a big chunk out of batoning firewood, RAT made good on it IIRC.
I see a whole lot of broken knives in the Army, Soldiers as a general rule horribly abuse their steel. Besides my own, I don't ever think I have seen a Soldier's multitool without at least one tool broken. With that said, I have seen them throw their knives for fun, try to cut through rusted bolts, pry, dig and about anything else you can imagine and most of the knives used are 40 dollars or less.
It seems to me if breaking knives by routine use happened with any frequency at all we would hear more about it on the forum. Chris