In the first few seconds of Communication Studies (S01E16), Professor Slater, during a conversation with Jeff, mocks on Britta’s name, saying Beetlejuice. In Cooperative Calligraphy (S02E08), when Troy asks Jeff what’s his underwear made of, he answers that they’re an “organic soy-cotton blend” and Britta quickly quips, saying that he usually wears “stripey turquoise Beetlejuice numbers” underwear. On the 1988 movie, if you say Beetlejuice three times, the ghost manifests. During season 3 production, Megan Ganz noticed that the word had been said on the show 2 times and warned Dan Harmon about it, which led them to add the word once more in Horror Fiction In Seven Spooky Steps (S03E05). Once Annie says the word, you can see a guy dressed as Beetlejuice walking in the background.