boned in the back
Similar to being stabbed in the back, except some people like that sort of thing - they just wish they had some prior notice first.
"Man, you really got boned in the back when your girlfriend cheated on you with her best friend."
"Yeah, the least she could've done was videotaped it so I'd have something to watch tonight."
"Yeah, the least she could've done was videotaped it so I'd have something to watch tonight."