1. In Captain Holt's office, his binders are arranged to display the colors of the pride flag — the binders are later placed in the same order behind Gina.
2. After Vivian breaks up with Boyle, he knows it's officially over when she gives him his dehydrator back — Gina does the same thing after she and Boyle stop sleeping together.
3. In the episode "HalloVeen," you can spot a cute photo of Amy and Jake on Amy's desk — the picture is actually taken from Melissa Fumero's Instagram.
4. Terry's portrait of Captain Holt can be seen hanging in Kevin's office at Columbia University.
5. Captain Holt wears a WTC breast bar indicating that he was working on 9/11.
6. Gina reveals that after Amy has had four drinks, she becomes more sexual. Later, when Jake and Amy go on their first date, they both order four shots and end up sleeping together.
7. While in the witness protection program in Florida, Holt keeps his lawn in pristine condition, while Jake’s lawn is a mess.
8. In the background of this scene, you can see that Hitchcock and Scully have created a system to pass papers to each other without moving.
9. When Captain Stentley arrives, we learn that he can't work the blinds. Later in the episode, you can see the blinds to his office aren’t closed properly.
10. And, after Captain Stentley leaves, you can see that he left his bongos behind in Captain Holt's office.
11. In Season 1, Captain Holt gives Boyle two puppies after his accident. In Season 2, the puppies re-appear when Boyle wants Jake to wash them.
12. When Boyle and Jake are on a stake out, their rule, “no peeling wallpaper,” is written on a section of peeling wallpaper.
13. Gina tells Charles that she would rather be hit by a bus than read another one of his texts. Later in the episode, Gina is literally hit by a bus after reading a text from Charles.
14. When Captain Holt and Terry visit Rosa in prison, Captain Holt ends up recreating an iconic moment from Lost:
15. When Amy meets her celebrity crush, Melvin Stermley, he's played by David Fumero, Melissa Fumero's husband IRL.
16. The game Gina and Captain Holt become obsessed with, Kwazy Cupcakes, is actually an app you can download IRL.
17. In Season 4, Captain Holt mentions that there are two acceptable sleeping positions — in Season 5, you can see him executing one of them.
18. On Gina's desk, you can see that she has a jar that reads "Ashes of Problem Employees."
19. In Season 1, Jake makes Amy wear a dress that reminds him of his first crush, Jenny Gildenhorn. In a flashback later on, you can spot Jenny wearing the dress.
20. On the wall in the conference room, you can see that Scully was given the detective award from the Association of Polygraph Examiners.
21. Patton Oswalt plays Fire Marshall Boone — a nod to the Brooklyn Nine-Nine producer Marshall Boone.
22. In the first Halloween heist episode, you can see that Captain Holt keeps his sandwich in his safe.
23. And finally, a detail revealed behind-the-scenes: some of the props used in Parks and Recreation have also been used in Brooklyn Nine-Nine.
Love Brooklyn Nine-Nine? Check out these posts:
31 "Brooklyn Nine-Nine" Moments That Are All Funny, No Filler
Which "Brooklyn Nine-Nine" And "Parks And Rec" Characters Are You A Combo Of?
23 Tumblr Posts You'll Only Understand If You're Obsessed With "Brooklyn Nine-Nine"
21 Times Jake And Amy's Love On "Brooklyn Nine-Nine" Made Us Melt