"The Underdome" in Mad Moxxi's Underdome Riot DLC is an obvious reference to the movie Mad Max: Beyond Thunderdome.
Contributed by TheMicroFreak
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In the Secret Armory of General Knoxx DLC location, Moxxi's Red Light, there are several references to the other DLCs behind the bar. There is a brain, Hank Reiss' hat, and a Jack-o-Lantern which reference the first DLC, The Zombie Island of Dr. Ned. There is also a model Claptrap with a moustache similar to that of Dr. Ned and a Marcus bobble-head.
Contributed by TheMicroFreak
The two enemies, Skagzilla and Mothrakk, are references to the Kaiju from the Godzilla movies, Godzilla and Mothra respectively.
Contributed by ThisGuyInTheSuit
Within the game's files is a video file that was likely a trailer for the game. Never shown before or after the game's release, the video features the song "Greenfields" performed by the Brothers Four, as well a few small differences from the final game such as an early model for the character Lilith, and "Fyrestone" being spelled "Firestone".
Contributed by Shadowmane
There is a boss monster called Rakkinishu. Its name is a reference to a carver boss from Diablo II named "Rakanishu".
Rakkinishu will always drop a shield called "Cracked Sash", which shares its name with the lowest quality belt item in Diablo II.
Contributed by Psychospacecow
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Borderlands originally featured a more realistic art style. Early screenshots and the game's initial reveal trailer show a lack of cartoon-esque black marks on textures.
Contributed by Funland47
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When Baron Flynt is killed he will drop a unique shotgun called 'Boomstick'. Its info line states 'Beyond Groovy'. This is a reference to Bruce Campbell's character Ash in the movie Army of Darkness (1992). Ash refers to his shotgun as his Boomstick and frequently uses the expression 'groovy'.
Contributed by cecil_banon
In New Haven there is a reference to the 1983 film, A Christmas Story. It can be found in a secret room which can be only be accessed during "Another Piece of the Puzzle". Inside is a Christmas tree, a leg lamp (made from a real leg), and a rifle called "Rider," with a caption which reads, "Careful... you might put someone's eye out".
Contributed by FUS RO DAH
The "My Brother Was An Italian Plumber" achievement or trophy's description is "Kill an enemy by stomping on them", which is a reference to Mario.
Contributed by FUS RO DAH
The game was originally advertised to have "87 Bazillion" types of guns (Obviously a fictional number). The actual amount is 16,164,886. Gearbox's Randy Pitchford had previously stated the number was around 17,750,000, somewhat close to the actual gun count.
Contributed by DidYouKnowGaming