1.First, in "Halloween," Amy spots a Raggedy Ann drinking vodka straight from a bottle — in Parks and Rec, Ann is dressed as Raggedy Ann and drinks vodka from a bottle.
2.When Holt enters the code to his safe, the scene cuts to it opening after he punches in "3, 1, 9," so even we don't know the full four-digit combination.
3.Also, Holt hilariously keeps his sandwich locked in his safe — probably so Hitchcock and Scully don't eat it.
4.In "Halloween," on the whiteboard behind Terry, you can see that there's a quote by Charles Barkley.
5.You can spot a flyer for Gina's dance troop "Floorgasm," which always has a performance on Halloween.
6.In "Halloween," before Jake reveals that Rosa and Terry swiped Holt's phone, you can see both Rosa and Terry brush past Holt during the royal baby distraction.
7.In "Halloween II," you can spot Rosa and Amy exchanging a look when Jake thinks he's won the heist. This is before it's revealed that they're working with Holt.
8.After taking Jake's wallet, Hitchcock says he can't get his bear costume's head off — at the end of the episode, he still has it on.
9.You can see Jake's police badge go missing after this interaction on the party bus — again, this is before we find out that the partygoers are Rosa and Holt.
10.In "Halloween III," after Scully leaves the blinds cracked, they're actually back to normal when Holt asks Amy to leave — it's a small continuity error.
11.In "Halloween IV," when the gang is forced to work the night shift, the name plates on their desks have been changed, since different people work during the day.
12.The only Halloween decoration Holt has in his office is a bat — during Season 3 he compared Wuntch to a bat too.
13.In "Halloween IV," before it's revealed that Rosa was dressed as a pizza delivery person, you can see her amongst the crowd.
14.You can spot a flyer for the zombie pub craw at Shaw's Bar on the bathroom wall behind Amy.
15.In "HalloVeen," Amy has an adorable photo of her and Jake on her desk — the picture was actually taken while filming the Season 4 finale.
16.On the maintenance schedule, one of the technicians is named B. Denton — this is a reference to Buffy Denton, who is the set dresser on Brooklyn Nine-Nine.
17.You can see a pair of wind-up false teeth next to Jake's bed, and then another pair on his desk.
18.When Jake is looking at security footage, you can spot Amy coming back from taking the cummerbund and Cheddar.
19.During the flashback to when Jake and Amy meet for the first time, you can see that Capt. McGinley works at the nine-nine — he's the captain that Holt replaced in the pilot.
20.While Jake's proposal to Amy was a huge surprise, you can clearly see the changed inscription before Amy reads it out loud.
21.And finally, in the beginning of "HalloVeen," Jake said that planning the Halloween heist got him through prison — and in the end, we learn that the Halloween heist was an elaborate proposal for Amy.