In beer pong or similar drinking games, the fuck-up cup is one cup which has substantially more beer than all the others. This often causes the partner of the pourer to argue that they don't have to drink it when that cup is taken during the game.
She: "... and then she said that we could go shopping at the mall. And so I said, that sounds like a good idea. So she drove us there, but then we could not find a place to park so we decided to go get something to eat..."