All original cast members from the 21 Jump Street (1987) television series have cameos in the film, except Richard Grieco and Dustin Nguyen. In one scene, the bad guys watch the original television series on television, and almost every shot shows Nguyen's character. Grieco and Nguyen later cameo in 22 Jump Street (2014).
While Jenko and Schmidt are being briefed on H.F.S., a video sharing site is on the computer. Two of the related videos in the sidebar were uploaded by users Chanchan and Brielar, references to Channing Tatum and Brie Larson.
Throughout the film, as the two main characters return to "The Chapel," they encounter two female officers who tout their own exploits, including busting a prostitution ring, and busting a chop shop run out of a high school auto shop. Both of these cases were plots in season one of 21 Jump Street (1987).
The drug Schmidt and Jenko go undercover to look for is abbreviated H.F.S., which stands for Holy F*cking Sh*t. It is also the first phrase the teenager in the YouTube video says when he takes the drug.
Before the end credits, a montage of various images are shown. One image shows a man at a typewriter. This is a reference to the famous Stephen J. Cannell Productions typewriter logo, which was shown at the end of his television productions, including 21 Jump Street (1987).
While appearing on the Howard Stern Show, J.K. Simmons revealed that he was considered for Deputy Chief Hardy, but scheduling conflicts prevented him from being cast. Simmons said upon seeing the movie, that this role was one of the few he would like to have played for the comedy.
When Schmidt and Jenko are planning their house party, Jenko jokingly says, "How are we going to get alcohol for the party?" to which Schmidt responds, "I don't know, I don't have a fake ID." This is a reference to Jonah Hill's storyline in "Superbad" when he's trying to get alcohol for the party at Jules' house.
The trivia items below may give away important plot points.
While undercover, Schmidt's cover is almost blown by someone he knows personally, and he avoids this by pushing her away and saying that she tried to grab his private parts. This same thing happened to Johnny Depp's character in Donnie Brasco (1997) while he was undercover. Depp plays Tom Hanson of DEA in this film.
Hanson appears in disguise as a biker eating a jar of peanut butter, based on a suggestion by Johnny Depp during his last season on 21 Jump Street (1987). Penhall also appeared in disguise as a biker, based on one character's dress in the original series. Jonah Hill specifically wrote the disguised characters for Depp and Peter DeLuise.
Johnny Depp's character, Tom Hanson, eats in every scene where he appears. Hanson ate often in the original 21 Jump Street (1987) television series, sometimes several times during each episode, and did so often in scenes vital to the plot.
At the end of the film, it is revealed that one of the bad guys is actually Tom Hanson, played by Johnny Depp, from the original 21 Jump Street (1987) series. He has been undercover for years with the villains using the alias D.B. Following 21 Jump Street, Johnny Depp played another cop who went undercover long term with criminals: Donnie Brasco (1997).
When Schmidt and Jenko take H.F.S., Jenko remarks that it tastes like Cool Ranch. Soon afterwards, they run into Coach Walters, carrying a bag of Cool Ranch Doritos. It is later revealed that Coach Walters is the mastermind behind H.F.S.
When the two undercover DEA Agents are gunned down, it actually conflicts with the original series episode 21 Jump Street: Back from the Future (1990), which features two elderly, retired officers reminiscing about past cases (Clip show episode).