I'm confused. I don't understand how it can be a surprise and hurt, because when that has almost happened to me (and it has happened on many occasions), the only way for it to actually go in would be if I was relaxed enough there, or if there was enough lube on the proper parts already, and even then I still had enough of a warning to say "Hey, stop, you're about to go in the wrong spot" xD
If I wasn't relaxed or there wasn't enough slip and slide going on, I don't think that would even happen