When Gru enters El Macho's fortress as a 'prisoner', he says, "Curses, foiled again!" That is the catch-phrase of Snidely Whiplash from the 'Dudley Do-Right of the Mounties' cartoon, and has been used by various other villains in other cartoons.
When Lucy and Gru are reviewing Paradise Mall shop owners (suspects), the owner of Stuff-A-Bear (a satire of Build-A-Bear) overstuffs a bear and it explodes, making an expectant little boy cry. The boy is the same one that Gru made cry with a popping balloon in the beginning of the first Despicable Me (2010).
When Lucy Wilde gives Gru a phone number to reach her, it is clearly visible as 626-584-5723. Calling that number reaches a recording of Kristen Wiig in character, with her outgoing voice mail message. However, the voicemail box is full, and you cannot hear her recording.
The actors are filmed the entire time they read their parts. Their facial and hand expressions are used on screen. Steve Carell's wife had fun pointing out the expressions in Gru that reminded her of Steve.
When Silas Ramsbottom shows Gru a video of a rabbit being injected with mutation serum, the name tag of the scientists in the video read "COFFIN" and "RENAUD" - a reference to directors Pierre Coffin and Christopher Renaud.
When Gru first visits his Minions in the underground workshop, several Minions can be seen taking a break and eating lunch on top of a cross beam. This is an homage to the famous "Lunch atop a Skyscraper" photograph from 1932 by Charles C.Ebbet.
The start of the film features yellow-jacketed humans speaking a foreign language. This is a reference to the original design for Despicable Me (2010)'s minions - the iconic shortened shape came about as a result of a low budget for the first film.
The 626 area code is from California, meaning that this movie and the original Despicable Me movie probably take place in the San Gabriel Valley or nearby in the northeast portion of Los Angeles county.
When Gru is spying on Margo he hides behind a cactus plant, three lobes of which give him the nose and ears of Mickey Mouse and another one that gives him the nose of Squidward Tentacles from SpongeBob SquarePants.
At Agnes' birthday party at the beginning of the movie, some kids are playing a fishing game in a small kiddie pool. One kid hooks a piranha that looks just like the ones in Vector's piranha gun in the first movie.
There's a brief shot in the film in which Edith is playing ping pong with a minion, but uses a pair of nunchaku as opposed to a ping pong paddle. This is a reference to a famous Chinese Nokia commercial in which a Bruce Lee impersonator in a mock "lost home video" also plays a game of ping pong using only a pair of nunchaku.
When the Minions assemble to give Dr. Nefario a 21-fart-gun salute, the bugle call "To the Color" can be heard playing in the background. This is a U.S. Army bugle call that is traditionally played on Army posts after the call of "Retreat," as the post flag is lowered at the end of the duty day.
When Gru enters the Eagle Hair Club and Eagle-san greets him, Eagle-san is petting what appears to be a long-haired white cat (which actually turns out to be a wig). This is a reference to James Bond films in which the leader of SPECTRE is petting a white cat.
When Gru takes off on his motorcycle (shades of James Bond) to rescue Lucy, he's wearing a helmet with goggles. The directors didn't want to show him wearing the goggles, because he would look like a Minion; plus, it's a throwback to various World War II movies where a hero rides a motorcycle.
One of many examples of how macho the villain is, the bull above the front door of the Salsa & Salsa was an actual bull. When Gru, Agnes and Edith enter the restaurant looking for Margo, you can see the back half of the bull protruding from the wall above the door.
When Gru goes back to Macho's house to rescue Lucy, he goes in handcuffs with two minions painted purple. The scene is reminiscent of R2D2 and C3PO going to Jaba's palace because of the shot of the door opening, with large teeth on the bottom of the door to prevent it from being battered in. It is shot from the same angle and is thus a tribute to Return of the Jedi in that brief shot.
When the villain tap dances on the floor tiles to enter the code for the door to his lair, after he repeats the five-note pattern a second time (first four measures of the song La Cucaracha) the final sequence is completed by the mechanism as a low pitched horn sound. This is how the alien ship responds when a five-note pattern is played over and over by the scientist in Close Encounters of the Third Kind.
This is the second time we find a Kristen Wiig character in a cupcake bakery. In this film, she has a scene with Gru in "Bake My Day". In Bridesmaids (2011) she used to own "Cake Baby"* bakery where she made cupcakes.
In Despicable Me 2 (2013), Gru (Steve Carell) is a single dad with three orphaned daughters, one of whom - Margo - is 'dating' a young boy, which Gru is trying to prevent from dating. In a previous film, Dan in Real Life (2007), Dan (Steve Carell) also plays a single (widowed) dad with three daughters, one of whom - Cara - is also 'dating' a young boy, which Dan is trying to prevent from dating. Interestingly, both of the young boys are Spanish-speaking.
The trivia items below may give away important plot points.
El Macho drinks a liquor bottle during a flashback scene. The label says "La Venganza del Escorpión", which is Spanish for "The Revenge of the Scorpion". Behind it there is a poster, also in Spanish, that reads "If you are drinking to forget, pay first."
One of the minions in the end credits has Heterochromia iridum. His left eye is brown while his right eye is green. This is Bob, one of the minions in the upcoming film Minions (2015) about how the minions met their boss.
At the wedding the minions singing "I Swear" were modelled to resemble the singing group Boyz II Men, however, the R&B group that originally recorded the song was All-4-One. Boyz II Men never recorded it.
When the minion with the Popsicle wakes up on the island, he looks around and then heads to the beach. When you look toward the water, there is a green "El Macho" flag visible in the sand on the left side.
Gru seems to be very wealthy, as indicated that he pays each of the minions wages, and there are over 10,000 minions. Although, it is also indicated that much of his money comes from the bank. However, his source of income in Despicable Me 2 is still unknown.
Whenever the Lipstick-Taser is tasing a person, It is noticed that the person dances rapidly fast. For example, when Lucy tases Gru he dances with his finger up and down and when Gru tases Eduardo/El-Macho he seems to be first dancing with a guitar then dancing with his hands over the waist before falling down.