In Episode 25, Setsuna and Lockon can be seen from behind, just in front of China going to watch the 8th Gunpla Championship Qualifiers.
In the prequel manga Build Fighters: Amazing, the main characters from the manga Gunpla Builders Beginning D, including goth-lolita styled Saki Mishima, show up as Julian's opponents in the Gunpla Academy.
Sei always imagines himself wearing Kira Yamato's Earth Alliance pilot suit.
Mao always imagines himself wearing Garrod Ran's civilian clothes.
Reiji (and later in the series, Meijin Kawaguchi) has a pair of sunglasses based on Char Aznable / Quattro Bajeena's from Mobile Suit Zeta Gundam.
Episode 20 has Sei quote Amuro Ray's famous lines from the infamous Bright Slap scene (comparison between both scenes can be found here◊). That was just the icing on the Offscreen Moment of Funny for the said ep: after the hypnosis session, Nils mentions that Sei had quoted all of Amuro's lines from ALL 43 EPISODES of the original during his hypnotized state.
GM Sniper K9: GM Sniper II with a play on the Seravee and Cherubim autonomous backpack. And the EXAM System from Blue Destiny just for good measure.
Hell Zeong Marine/Galaxy: A cheaper but more versatile version of Perfect Zeong.
The V2 Gundam piloted by the Fonse Kagatie look-a-like in episode 14 is repainted in Zanspine colors.
Aila's Miss Sazabi is a combination of the Sazabi body and arms, Nobel Gundam legs, and Infinite Justice feet.
Yuuki's Exia Dark Matter, which takes the Exia, mixes in a Dual Wield aesthetic from the Strike Noir and Beginning J, and further blends in elements of "The Strongest Enemy", from the Zeong, Providence and Reborns. The Dark Matter Booster's design itself is reminiscent of Sid.
For the final episode, Yuuki's Amazing Exia Repair, which is essentially a kitbash of the Amazing Exia with Kampfer Amazing parts and the Exia Repair's cloak.
Master Chinnan, Ral and Takeshi also sport slightly customised versions of the Master Gundam, Gouf and Perfect Gundam.
Also, Ral's Gouf is the Gouf R35. A nod to how Ramba Ral in Mobile Suit Gundam and Build Fighters is 35, as well as the 35th anniversary of Gundam.
According to the HG model manual, the Gouf R35's booster pack is based off of the one Ramba Ral used when he led a commando raid on White Base also the same battle in which he died.
In the epilogue, Mao and Chinnan are seen playing around with two heavily kitbashed models, actually real life customs made by Mikako Komatsu and Sachi Kokuryu (aka Sei and Reiji's voice actresses) to promote Gundam Breaker, a new PS3 game that's basically Build Fighters: The Game (albeit against AI opponents).
In the 1st opening, Build Strike mirrors Strike Gundams pose from the 1st and 2nd openings of Mobile Suit Gundam SEED.
When Yuuki participates in a Gunpla Battle, he lifts his hair back to show his forehead, not unlike H/Allelujah Haptism in his final battle of the first season in Mobile Suit Gundam 00.
A model of the Dijeh, the Mobile suit Amuro used in Zeta, is shown painted in the classic Gundam colors.
In episode 1, the damage Sazaki does to Sei's Wing Gundam is identical to the damage it takes after Heero self-destructs the unit early on the series.
Whenever Fellini manages to successfully pick up a girl, the beam rifle sound effect plays in the background.
Reiji's battle with Fellini has a brief moment where the Wing Gundam Fenice looks up and its eye flashes; this one's a two-fer, imitating the original Wing and Wing Zero in each of Gundam Wing's openings.
Reiji takes out 12 G-Bits with ease, referring to the famous battle from Mobile Suit Gundam, where Amuro takes on 12 Rick Doms at the same time.
The clock in Sei's room has "Anaheim" on the face, referencing Anaheim Electronics from the UC timeline.
The title of Episode 8 "Ai Senshi tachi (Encountering Warriors)" shares the same pronunciation with "Ai Senshi (Soldiers of Sorrow)" the Ending song of Mobile Suit Gundam.
The Build Gundam Mark II taking out the Apsalus III is reminiscent of the climax of the 08th MS Team OVA, due to how both Gundams (Build Mk-II and Ez8) are piloted by two people and the fact the Build Mark II got in close for the final blow.
Yuki's alias Kawaguchi is a reference to Katsumi Kawaguchi, an experienced gunpla builder who works for Bandai. As of November 21 2013, his facebook profile has Cool Shades like one in the show.
There are several location shout outs given in Episode 8.
The La Vie en Rose Garden, the venue for the Gunpla World Championship America Qualifiers, to the LaVieenRose. It can also be referred to New York's Madison Square Garden or Portland's Moda Center (formerly known as the Rose Garden).
One of the fans is wearing the trademark Char Mask (Though it looks like Zeheart's)
Rivals Logan and Fellini use modified versions of rival suits the Tallgeese and Wing Gundam from Mobile Suit Gundam Wing. Incidentally they are friendly rivals.
The Qubeley Papillon and the Alpha Azieru during the Finnish National Finals are a subtle nod to the fact that both of the suits are piloted by girls who are interested in Char Aznable (Haman Khan and Quess Paraya respectively).
The two customers at China's family restaurant, the Marasai team from another episode, are wearing T-Shirts with the Titans uniform patterns from Mobile Suit Zeta Gundam.
Caroline's Knight Gundam when it got caught in the Beargguy III's cotton resembles Zero the Winged Knight when he got put in the rice bag in SD Gundam Force.
The Beargguy III's cotton-filled armor and "weapon" may be a reference to Mobile Suit Crossbone Gundam's Tortuga, which contains fast-hardening gas that traps any enemy capable of penetrating its armor.
China gives Sei and Reiji good luck charms. Sei's is a classic green Haro, while Reiji's is an orange one.
Alia and the meat bun is a reference to Allenby's love of meat buns from Mobile Fighter G Gundam. Alia is even somewhat similar to Allenby.
At the end of it's debut battle, it also makes the same noise as the GN-Drive.
The Mega Size Zaku makes breathing noises, recalling the first scene of Mobile Suit Gundam where the team of Zakus infiltrating Side 7 made similar noisesnote It also doubles as a Shout-Out to Star Wars, since it sounds nearly identical to Darth Vader's breathing noises.
The recolored Zeus Gundam tries to finish off Gundam X Maohin the exact same manner the original Zeus Gundam tries to finish off God Gundam in "G Gundam".
The race battle is a reference to "SD Gundam Crazy Race" episode in old SD Gundam Mark IV OVAnote which is a homage to Wacky Races.
The battle between Sei/Reiji and Fellini ends up referencing Char's Counterattack and Gundam Wing more than once; firstly, Sei sets up the Universal Booster with a wire to serve as a "trap", just like Amuro did with the Nu Gundam's bazooka.
Secondly, the fight sees the Strike and Wing gradually whittle each others' weapons down until they're left bare-knuckle boxing with each other, as happened with the Nu and Sazabi.
Fellini using his rifle to blast the terrain may be a nod to Heero's Antarctic duel with Zechs, where he likewise shot up the terrain to gain an advantage.
Not only that, but this happens after they try and fail to destroy the Kampfer by attaching explosives to it - again, nearly the exact opposite of what happened in War in the Pocket. The main difference is the EXAM System activation in between.
The Renato brothers' secret plan, Operation Time Stop, is a reference to the original series episode "Time, Be Still"; both involve Zeon soldiers planting bombs on their enemy's mobile suit.
The title of Episode 20 "Uragiri no Aira (Aira's Betrayal)" is a play on the ∀ Gundam episode "Uragiri no Guen (Guin's Betrayal)".