This is a big fantasy of mine (either female or male) but unfortunately I've never experienced it. It would have to be fresh though, not from some random unknown person and not knowing how long it had been in there. A colleague at work complained of pulling a girl in some trashy club in Bristol and going down on her in an out of the way corner only to find she was already full of cum (must have been a classy venue).
The closest I've got is causing a situation like this. I had a friend that I'd have sex with fairly regularly, he had a boyfriend I didn't like as he bullied me at school (not that it makes it much better, but at the time I was fairly foolish and didn't really care). One day we had sex when he finished work while he waited for his boyfriend to come and pick him up, but unfortunately for him his boyfriend noticed the cum still seeping from his ass when they got home.