This has happened to me twice.
Once during my first blowjob, the girl was very clear that I was supposed to warn her before it happened. I warned her, I swear I did. I think I did.
She was not happy, but it wasn't as bad as she thought and after that she would swallow me whenever she gave me head. Of course that started to taper off after a year or so.
The other time I was in the cooch. Same girl. We only had one condom and I screwed it up. Took it out, tried to put it on backwards, dropped it. Finally she said to just please fuck her and leave it off, but don't come inside. No problem, I'm a master of control. We were going at it for a long time, ten minutes missionary, ten minutes doggy style... the whole time I'm thinking "Don't come in her, don't come in her". Finally she turns around and starts riding me. I told her "Get up" but she thought I meant sit up. She sat up on me straight and I blew inside. She went ballistic. I tried to tell her it was her fault but that just made it worse. She kept saying she was going to get pregnant.
She made me buy her a pregnancy test (that night, regardless that I told her it wouldn't work for weeks and it even said so on the box). I bought her three more pregnancy tests, in fact I think they were two-packs so she must have peed on eight of them and they're not cheap. She was late, but only a couple days. A couple nerve wracking days. This led to a fight and we broke up. She wasn't pregnant.
Ever since then I've developed a fetish for the whole concept of forbidden cum, though I've never done it again myself.