Community has pulled off one the most patient easter eggs ever.
In one episode of each of the first three seasons, the word “Beetlejuice” was used off-handedly in a joke. If you’ve seen the movie Beetlejuice, the titular mischievous ghost would appear in the world of the living if anyone said his name three times. So, sure enough, on the third mention by a Community character, this guy appears in the background for exactly two seconds. They patiently waited three years to reach that punchline.