I've never heard of someone firing the wrong round in a rifle. I bet that was all kinds of interesting. At least the case sealed up the chamber for the most part, you wouldn't want that kind of pressure coming back at you.
We see it almost every weekend. I've seen .300 Win Mag shot in a .300 Weatherby Mag, and .270 Weatherby Mag shot in a .300 Win Mag. Alot of .380 in 9mm, some .32 in .380s, .357 SIG in .40, even a .40 shot in a .45. We keep a collection of random "oops" brass at the store.