important social issues, such as racism, homophobia, and sexual harassment.
Some fans may not be aware of this, but the writers make sure to include hidden details and easter eggs within the episodes. While most people have spotted the hidden details of the precinct, not a lot of people have spotted the hidden references to plots of the previous episodes. Just look at these for example...
Boyle's Dehydrator
In season 1, Charles gets engaged to Vivian, a fellow food enthusiast. However, when Charles finds out she wants to move away to Canada, he realizes he has to break off their engagement. While giving each other their stuff back, she hands Charles his dehydrator, and a teary-eyed Charles says it's official.
Later on, during season 2, when Charles and Gina are hooking up, Jake finds out about it, and they decide to break it off. In a later episode, Gina gives him back the very same dehydrator, signifying the end of the relationship.
Captain Holt's Portrait
In the season 1 episode, "M.E. Time," Amy sees that Captain Holt is in a terrible mood and asks Terry to paint a portrait of him to cheer him up. However, far from appreciating the gesture, Holt is indifferent to it. Later, Amy correctly deduces that Holt is anxious about the crime statistics not improving, and manages to comfort him.
Holt notices Rosa likes the painting and agrees, saying that he'll give it to Kevin. Later, in the season 1 episode, "The Party," when Jake pays a visit to Kevin at his office in the University, the same portrait is seen hanging behind his desk.
Four-Drink Amy
In the season 2 episode, "Beach House," the squad goes on a vacation at a beach house, and Amy is packing drinks into her luggage. Gina sees her and asks her if this is the weekend she'll finally get to meet Six-Drink Amy. Gina explains the Santiago Drunkenness Scale and explains the various stages.
She says that Four-Drink Amy is usually horny and a 'little bit of a pervert.' Later on in season 3, when Jake and Amy are on a date, Amy is exactly four drinks down when she and Jake sleep together, thus confirming what Gina had said.
Boyle's Dogs
In "Pontiac Bandit," Holt is looking for people to adopt his two puppies, Richard and Dan. When nobody is willing to adopt them, he decides to give them to Boyle to help take his mind off his recent injuries, when he bravely took a bullet to save Rosa. Charles is quite happy to take them.
We see them again, albeit as fully grown dogs, in the season 2 episode, "Payback." Jake decides to work off his debts to the various squad , and Charles tells him to wash his dogs.
Holt's Sleeping Positions
In the season 4 episode, "The Last Ride," after finding out the Nine-Nine might be shutting down, Holt decides to mentor Santiago in one day, instead of his planned nine years. He gives her advice on various topics, such as adults who chew bubblegum (never trust them), the best type of shirt collar (English spread, obviously), and vacationing in Banff (don't).
He also tells her the two acceptable sleeping positions - on back, toes up, arms crossed, or arms on the side. Later, in the season 5 episode, "99," when Captain Holt is sleeping in the Boyles' house, he can be seen sleeping on his back, arms crossed.
The Shoulder Nova
In the season 3 episode, "Into the Woods," Amy takes Gina's help with marketing her business idea - a flashlight strapped to her shoulder, which for which Gina suggests the name "Shoulder Nova." And although the idea is rejected, the Shoulder Nova makes another appearance in the series.
In "Chasing Amy," Amy is using the Shoulder Nova to study for the sergeant's exam while in bed, in a blink-and-you-miss-it flashback. It was a great callback and a fun detail by the writers which fleshes out the world of the show.
Rosa and Arlo
In the season 3 episode, "9 Days," Charles is mourning the death of his dog, Jason. Rosa doesn't understand how Charles feels, but wanting to do something anyway, gets Charles a new puppy named Arlo. However, Charles doesn't want to replace Jason, and when Rosa forms a bond with Arlo, she decides to keep him.
The series mentions Arlo a few times after that, like when Rosa asked Holt and Terry to bring him to Pimento in Argentina while she in jail. She also mentions how much Cheddar comes to the precinct, saying she never brings Arlo. When Jake asks who Arlo is, she replies with "Exactly."
Amy Dresses Like Jenny Gildenhorn
In "The Bet," when Jake gets to take Amy on a terrible date, he makes her wear a fluffy blue dress, saying it was the same kind of dress Jenny Gildenhorn, his childhood crush, wore during his Bar-Mitzvah.
Later on, in "Charges and Specs," a flashback shows Jenny breaking up with Jake at his bar mitzvah. Eagle-eyed viewers were quick to observe that her dress was, in fact, the exact same dress that Amy wore during their date.
Holt Maintains His Lawn
Throughout the entire series, Captain Holt has been shown to be extremely neat, tidy, and well-kept, a quality shared by his possessions. He is very well organized, and his love of rules initially alienates him from the childish and untidy Jake, although the two end up becoming extremely close.
In the season 4 premiere, "Coral Palms," Jake and Holt are both in witness protection hiding from Jimmy "The Butcher" Figgis. Fans can see quite clearly that the meticulous Holt has taken great effort to maintain the lawn of his temporary residence, whereas Jake has made no such efforts.
CJ Can't Close The Blinds
In season 4, Captain Holt gets replaced by Captain Jason "CJ" Stentley. He is shown to be quite clumsy and dumb, as Terry tells the squad that he saw CJ spill hot cocoa all over himself, and couldn't figure out how to close the blinds. So, he just changed his pants with the windows open.
Later on, fans can see the blinds to the office aren't closed properly as CJ is playing the bongos. Speaking of the bongos, fans can see he has left them behind on a shelf in Captain Holt's office after he is transferred.