An apology for any acts that you may (or may not) remember after a bout of drinking/partying. While not required, it is almost always accepted by the receiving party.
It is almost always said while hungover.
Friend 1: "Dude, you left the club last night with my sister, and banged her. Then when you were leaving you stumbled in my mom's room and had sex with her too!"
Friend 2: "Sorry for party rockin'"
Friend 1: Oh when you put it that way...it's cool