Noun. When one mistakenly shacks (shares a bed/spends the night romantically, the act of sex isn't a shack requirement) with someone for an evening, whether it was a drunken mistake or turns ugly later. The term can be used to refer to the incident itself or the person shacked with.
I had a shaccident. I shacked with that guy, and now he's clingy like a parasite. OR That was a total shaccident. I didn't realize how ugly she was until this morning. OR He's a shaccident. I meant to shack with his friend instead. OR She's a shaccident. She was a terrible kisser.