Sorry, but I have to call bull shit here, with a side order of stupidity. It's not like rape. The original post state he accidentally put it in her ass. Last I checked, it was nearly impossible to accidentally rape someone. Seriously? There's something called 'respect' that should be employed more in some of these posts.
You'd be surprised at what can happen accidentally. One time I was drunk and my wife and I were 'in the moment', and in a hurry to get going. Have you ever tried to figure out which way a condom unrolls in the dark when you're drunk? Needless to say, in 1.2 seconds I had had enough, and simply shoved it down on my cock, hard. Wasn't exactly the brightest move I ever made, but hey - drunk here! Or there, I mean. Anyway, it was moderately painful for me, and it had really created some serious suction. I couldn't get the damned thing off until I yanked several times. It stretched out like a rubber band and finally snapped off with a loud "POP!". I fell off the bed when it came loose, and we both laughed ourselves silly. The point is, some really odd things can happen, so don't jump to conclusions too quickly. Troll comment was kind of rude, too.
Only those who have been there and done that know it can happen, Just like who shot Kennedy, only the ones involved know the truth and the rest just have our half ass opinion, which proves nothing. But if you were there and you know what happened they say you are full of shit...
This does happen accidentally, but very rarely. I've seen it happen during a 4some. The woman lifted her head quickly and yelped, and they bumped heads. It was funny. His cock was slightly short, so I think it was easier for him to pull all the way out of her pussy. I've also seen at least two porn blooper out takes where it happened, but they were fucking really fast which made having an accident easier.
It happens accidentally, but most of the times it is called an accident... well... it wasn't. It was the dude being a douche. Once is not a problem. If it happens more than once, then the relationship has more problems beneath the surface and you need to get out of it.