I was having sex with my girlfriend's sister, and her mom walked in. Not only did she start screaming because I was piping her daughter, she knew I was going out with her other one so she went absolutely nuts and got on the phone. I knew she was calling my gf to tell her what had just happened so I panicked and ran out of the house after throwing on my clothes.
In order to save face, I called my gf while driving away and dumped her. She sounded really confused and hurt, turns out her mom handn't called her but her father instead. She learned eventually though.
That is one of the funniest things ive ever heard.