Dan Harmon's Community was one of the strangest sitcoms to ever hit the airwaves on NBC, or any network for that matter. The story of a group of students who spent six years at a terrible community college when the programs were only two years long got really weird. But the most absurd part of the series was Abed's obsession with the show Cougar Town, and the fact that the two series actually crossed over.
Abed Nadir had a difficult childhood, something that came up frequently on the series. His parents fought constantly, which eventually led to his mother leaving. While this was never confirmed on the show, it was heavily implied that Abed was on the autism spectrum. He had trouble connecting with people until he got to Greendale Community College. Abed's friendships with Troy, Annie, Jeff and the rest of the crew helped him greatly.
But before Abed got there, he expressed himself through his passion for movies, TV and video games. While he was at Greendale, Abed became thoroughly obsessed with a real world sitcom called Cougar Town. The series starred Courtney Cox as Jules Cobb, who was exploring life post divorce. While critics didn't always love this show, it built up a loyal enough fanbase to run for six seasons on ABC and TBS.
Abed's passion for Cougar Town was referenced more than once throughout the series, as he often mentioned the show and was concerned about it being cancelled at one point. But it was in Season 2, Episode 19 when things were taken to new heights. Abed had a birthday dinner with Jeff Winger where he detailed going to the Cougar Town set and actually being an extra in an episode. At the time, it seemed like a one-off gag on Community, but it didn't play out that way.
Episode 24 was the Season 2 finale, and was the second part of a two episode paintball war that the entire school had been involved in. As the whole battle came to an end, a crowd shot revealed that two actors from Cougar Town were there. Specifically, it was Busy Phillpps who played Laurie Keller and Dan Byrd who played Travis Cobb. But that wasn't the end of the crossover.
For anyone who also watched Cougar Town, they got to see the scene Abed described actually play out a couple of weeks later. In Season 2, Episode 21 of Cougar Town, Laurie and Travis were having a conversation when Danny Pudi appeared in an uncredited role as Abed. He sat behind them, staring at them while they talked and suddenly running out of the scene. The whole thing basically played out as Abed had detailed to Jeff.
The strange thing about this whole scenario was that Cougar Town was still a TV show in the world of Community. Danny Pudi did not appear as himself or another character on Cougar Town, but as his character from Community, Abed. Alternatively, it was Busy Phillpps and Dan Byrd who appeared on Community, not their respective Cougar Town characters. This was yet another meta layer added to a show that thrived on self-referential meta humor.
It was an unexpected crossover, which was kicked off by the running gag of Abed loving Cougar Town. There wasn't anything to cross-promote, and the whole thing wasn't advertised at all. This was just a couple of shows on completely separate networks having fun with an organic crossover that was just for the heck of it.