OK, so any viewer would have caught that on a second viewing, even if they missed it the first time around. But if you noticed this next one the first time around, there's probably some kind of medication you should be on: It's a "blink and you'll miss it" moment that took three seasons to set up:
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 (played by Michael "holy shit this guy used to be the biggest star in Hollywood?" Keaton) 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:
It could also just be that Michael Keaton was lost.
It's a random person dressed as either Beetlejuice or Gary Busey. That they patiently waited three years to reach that punchline is staggering A) because it looked like the show was going to get canceled several times before that, and B) when you consider that some shows can last eight seasons and the only punchline is "Look! Nerds!"
So how do you top that? How about by requiring the viewer to go chasing after the joke on a completely different show, on a completely different network? In a Season 2 episode, Abed tells a seemingly throwaway anecdote about getting work as an extra on the show Cougar Town (yes, that's a real show, on ABC) where he pooped his pants due to the stress. If fans had decided to switch over and actually watch Cougar Town, they'd sure enough have seen Abed showing up as an extra in the background, only to dash off at the end of the scene. You know, as if he had an urgent gastrointestinal emergency.
We haven't seen the show, but we're assuming everyone at that restaurant was attacked by cougars at the end of the scene.