And So We Commence (1) - S8-E24
Plot hole: When Theo is complaining that his father is inviting too many people to his graduation, he says "I saw him in the backyard 5 minutes ago hosing down the beach chairs", but this couldn't have been possible because in fact, Cliff was upstairs right before that scene, and then went straight to the living room, so there's no possible way that Cliff could have been in the yard at the time Theo had said he was.
Chosen answer: That is the song "Open the Door Richard."