But your brain will be damned if it remembers anything for your history finals.
And Community has some of these that are so well-hidden, they apparently don't care if more than a dozen people actually catch them the first time around -- to the point that some of these bits span across seasons, and even shows on other networks.
So, for instance, in one episode the character Abed has an entire storyline that plays out in the deep background of several scenes. At the beginning you see (through a window, behind the character who is talking) Abed befriending a pregnant woman. Later, while the main characters are having a discussion outside, you see Abed behind them, on the sidewalk, fending off the girl's jealous boyfriend. Finally, near the end of the episode, you see him delivering the baby itself in the parking lot behind the "real" scene that's going on in the foreground:
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: