1. The show is based on creator Dan Harmon's time at Community college, where he tried to get back with his ex-girlfriend in his Spanish class.
2. In "Physical Education" in season one, Abed does an impression of Don Draper from Mad Men when he pretends to seduce Annie. Alison Brie, who plays Annie, also plays Trudy Campbell in Mad Men.
3. The impression clearly had an effect on her, as she remembers it when Abed does another impression of Draper in Season 3.
4. Patrick Stewart was considered for the role of Pierce, which eventually went to Chevy Chase.
5. In "The Psychology of Letting Go", the main storyline is about Pierce trying to come to terms with the death of his mother. Abed is barely seen in the episode, but if you watch carefully he actually has an entire sub-plot in the background, where he ends up delivering a baby.
6. When Abed references this in "Applied Anthropology and Culinary Arts", and Troy asks him how he didn't know about it, Abed tells Troy he must have just been "off in the background".
7. When Jeff, Annie, Troy, and Abed are chasing down Professor Professorson in "Conspiracy Theories and Interior Design", they are blocked by a Latvian independence parade. The episode originally aired on 18 November, which is the day of the declaration of Latvian independence.
8. Each character has a real Twitter account, and they were used to live-tweet #AnniesMove, like Troy and Abed did in the show.
9. The third time Beetlejuice is mentioned in the show, by Annie in "Horror Fiction in Seven Spooky Steps", a man in a Beetlejuice costume walks by in the background, which references the Tim Burton film in which he appears if his name is said three times.
10. Magnitude is played by Luke Youngblood, who played Lee Jordan in the Harry Potter films.
11. In "Football, Feminism and You", Troy tells Jeff to embrace Greendale and "take a pottery class or something". Later in the season he ends up doing exactly that.
12. In "Aerodynamics of Gender" in Season 2, Abed's HUD that tells him how to insult the mean girls also foreshadows a number of the season's later episodes, including the paintball sequel, the Christmas special, and the blanket fort.
13. In "Contemporary American Poultry" in Season 1, Pierce tries to coin the phrase "streets ahead", which Dan Harmon put into the show after someone used it to insult him on Twitter.
14. In the Season 2 opener, Troy is seen wearing Spider-Man pyjamas, which is a reference to Donald Glover's "Donald4Spiderman" movement – when he tried to get himself cast in the 2011 reboot of the superhero franchise.
15. At the Valentine's dance in "Early 21st Century Romanticism", Starburns' sideburns are shaped like hearts.
16. In "Pillows and Blankets", there is a painting of English Memorial, the man who discovered Greendale. He has Dan Harmon's face.
17. Season 3's "Studies in Modern Movement" makes two nods to Friends. Shirley references the show early on, and later, Abed is seen wearing bubble wrap on his head while Troy hits him, just like Joey, Ross, and Chandler.
18. In "Critical Film Studies", Abed tells Jeff that he's been given a part as an extra in his favourite show, Cougar Town. He then appears in Cougar Town's Season 2 finale.
19. The boat Pierce bequeaths to Troy to sail around the world in is named "Childish Tycoon" – a reference to Donald Glover's rap career as Childish Gambino.
20. The music playing when Troy leaves in "Geothermal Escapism" is "Come Sail Away" by Styx. This refers all the way back to the eighth episode of the show, when Abed shows his short film to the group. In the film, Troy's actor is crying, and Troy asks: "Did I accidentally listen to 'Come Sail Away' by Styx again?"
21. In "Cooperative Calligraphy", you can actually see the monkey Annie's Boobs stealing Annie's pen at the very beginning of the episode.
22. There is a running theme involving Jeff's underwear. The ones that he wears in the episode where he plays pool are later seen on Dean Pelton in "Documentary Filmmaking: Redux", and the ones he wears in "Debate 109" are worn by Chang in "Early 21st Century Romanticism".
23. In "Contemporary Impressionists", when Troy realises Abed has lied to him, he says: "We made a deal. Oct. 15, 2009. Friends don't lie to each other". This refers to a deal they made during the fifth episode of the show, which aired on that exact date.
24. In "Cooperative Escapism in Familial Relations" in Season 4, Britta has Annie's pen from "Cooperative Calligraphy".
25. After the riot in Season 3's "Course Listing Unavailable", the group is seen in the hospital and Abed has a towel over his face. This is because a stand-in actor had to fill in for Danny Pudi so he could attend the birth of his twin daughters.
26. In Season 5, the characters frequently refer to the previous year as a "gas leak year", and is a way to explain how the characters weren't the same in Season 4. This is because creator Dan Harmon was fired after Season 3, but then brought back for Season 5 when Season 4 went down poorly with fans.
27. Pierce makes an appearance in Season 5, despite Chevy Chase making an exit agreement with Sony at the end of Season 4 that banned him from setting foot on stage. They got around this by filming him in a totally different location with a different team, and having him appear as a hologram.
28. Troy and Abed being sat next to each other at the table was a total coincidence, and the extent of their close friendship was not always planned.
29. When Annie and Shirley are hired as security guards in "The Science of Illusion", Shirley takes a pop at Annie after she fails to catch someone she's chasing down by calling her Hannah Montana. Alison Brie made her first acting appearance on that show.
30. In "Intro to Political Science", Pierce's age on his ID is listed as "66, dick" – this is a callback to a scene in "Advanced Dungeons and Dragons".
31. Pierce gets scared of the CGI Colonel Sanders in "Basic Rocket Science" in Season 2. When we meet Pierce's father in "Advanced Gay" in the following season, it turns out the two look extremely similar.
32. Pierce's father is named Cornelius Hawthorne. Chevy Chase's real first name is also Cornelius.
33. In "Basic Lupine Urology", Troy wears a Spider-Man tie, which is another reference to the "Donald4Spiderman" movement.
34. There are continuous references to an incident in Britta's past in when she was molested by a person in a dinosaur costume on her 11th birthday. It is first mentioned when she is in therapy with Professor Duncan in Season 1, and crops up again and again, most notably when Britta wears a T-Rex costume herself in "Epidemiology".
35. "Advanced Dungeons and Dragons" is Dan Harmon's favourite episode.
36. "BBT Sucks" (Big Bang Theory Sucks) appears in the background in a number of episodes.
37. In her speech to become student chair in "Intro to Political Science", Annie claims that "the ass-crack bandit will be brought to justice". This is finally made sense of in Season 5, when an entire episode is dedicated to the hunt for the ass-crack bandit.
38. The Dean has a strange but unsurprising obsession with dalmatians that subtly appears throughout the series.
39. The diorama the study group makes of the study room in "Paradigms of Human Memory" has a figure of Chang watching through the window from the outside.
40. In "Competitive Ecology", Chang is seen cutting up a newspaper. That newspaper has the headline "Inside Greendale's Hottest Pillow Fort" and is from "Conspiracy Theories and Interior Design".
41. The song "Daybreak" by Michael Haggins crops up subtly several times throughout Season 3.
42. When the City College flag is raised and the Greendale flag is lowered in "For a Few Paintballs More", it looks like this...
43. In "First Chang Dynasty" in Season 3, Chang remarks: "Fire can't go through doors stupid, it's not a ghost!" In "Paranormal Parentage", Pierce calls back to this, when he says: "Ghosts can't go through doors stupid, they're not fire!"
44. When Annie and Shirley break into the Dean's office in "History 101", there is a poster on the wall advertising a Shakespeare play by Professor Sean Garrity – Jeff's mystery teacher in "Conspiracies and Interior Design".
45. In "Remedial Chaos Theory", you can see Troy and Abed's fridge is decorated to look like a vending machine that has Troy in the bottom right, which is a throwback to the time the two got trapped in a vending machine in Season 1.
46. The Season 4 plot where Abed meets a friend from a forum who provides competition to Troy is the one Dan Harmon says he most regrets not being able to do.
47. In Season 5's "Introduction to Teaching", you can see a corkboard fall down as the angry students rampage into the cafeteria. This corkboard then becomes the focus of "Analysis of Cork-Based Networking" three episodes later, when Annie and Buzz Hickey try to get it put back up.
48. In that episode, a news ticker runs along the bottom of the screen while Hickey is watching TV that reads: "Levar Burton and non-celebrity companion captured by pirates in the Gulf of Mexico" – referencing Troy.
49. In "Abed's Uncontrollable Christmas", in which the characters are all clay models, they do in fact make one very subtle appearance as their regular human selves.