The scene where Derek tells Ricky that he has to go (after reading Ricky's love letter to Sally) has Derek making fun of Ricky by repeating the line in Ricky's poem "...I don't know why a cow goes moo". This causes Ricky to ball his fist and lunge at Derek which sends Derek falling backwards in his chair. It's pretty clear this was unintended as Ricky is almost laughing when the camera pans back to Ricky..