Having sex with someone when you honestly didn't intend to, or are trying not to. Not always a bad thing and is no big deal because 'hey, accidents happen'.
I went over to my ex's place to drop off some stuff last night. We got talking, one thing led to another and we had accidental sex.
When you don't really realize what you are doing and have sex with someone but it was a total accident and you didn't mean for it to happen.
"The other night i took her home from a party because she was cold and i had accidental sex with her, because I was pissed at the girl i was wheeling"