A fast one
Going to the bathroom or pretending to go to the bathroom so you can get drunk, do drugs, jerk off, or leave.
He said he was going to the bathroom, but he came back 20 minutes later with a full bag of chips and a guy.
She said she was going to the bathroom, but she came back with a new friend and a drink.
My boss said he was going to the bathroom, but he came back with a drink, a guy, and a new plan to steal my job.