Although very rare, a woman in Dallas gave birth to twins with different fathers almost a year ago. She was having an affair while engaged to her fiance and it seems there was an overlap between her travelling eggs and the travelling set of sperms, thus, producing the twins Justin and Jordan who are also, technically, just half-brothers.
I was not really surprised by this. I know for a fact that a man can even pass someone else's sperm to you (say, if they both had sex with the same girl) and thus get you pregnant with that other man's spermies. Also very rare but it CAN happen.
Anyway, hopefully, the twins will be raised impartially and that the mother will stop having affairs.