Fan Nickname: Some fans have taken to calling the Bullymongs "Bonerfarts", even going so far as to never finish the quest given by Hammerlock so they keep their names. Other fans also refer to Axton's turret as "the little lady" or "the missus" after the affectionate nicknames Axton himself uses when deploying it.
Various digs at FUNimation once the fans realized almost all of the actors listed above have done work for them at some point.
Jossed: A popular theory was that Krieg is actually the father of Tiny Tina. Anthony Burch later stated that both of Tina's parents are dead.
Memorial Character: Michael Mamaril, the generous Vault Hunter that sometimes appears in Sanctuary, and who was created in the memory of Michael John Mamaril, a Borderlands fan that passed away at the age of 22, and whose friend asked Gearbox Software for a short eulogy by Claptrap, Michael's favorite character.
No Export for You: Not the game itself, but several of its marketing tie-ins, including two contests and a pack of limited trading cards, are only available to Europeans, which is sort of odd for an American game.
On the other end of the spectrum, the companion app for the game, Loot the World, is only available in the US iOS App Store and Google Play Store. Not even the Canadian or Mexican stores has it. Only the United States. Given the title, it is apparent that Gearbox is that ignorant of world geography. Needless to say, players of the game from outside the US aren't too pleased, especially since Gearbox had promised that the app unlocks unique weapons not usually found by looting in-game.
The Other Darrin: Both Roland and Mordecai's voice actors changed hands - Roland now sounds less gangsta and more leader-y, while Mordecai has an accent.
Dameon Clarke (Handsome Jack's voice actor) ad-libbed a number of his lines, including the moment early in the game when he calls the Vault Hunter, mouth full of food, and mutters "These pretzels suck."
Anthony Burch: It's easily Jack’s single most popular line, and I didn't frigging write it. I will never forgive Dameon for this.
Character dialogue was meant to be targeted, meaning Axton would give flirty lines when reviving Maya, but neutral otherwise. After deciding against targeted lines, the Dev team realized that this means Axton can say lines like "Wow. Do you work out, or...?" to the male characters; despite this, they decided it would be better to just make him bisexual instead.
Tediore guns were originally going to be unreliable and have a chance of exploding in your hands. Tediore guns in the finished product are dirt-cheap and disposable, but perfectly reliable.
A bit Hilarious in Hindsight, since a minor oversight by the development team has made him Ambiguously Bi while a line in the Tiny Tina DLC has him remarking that he'd use his loot to get chicks and probably dudes. It's now been confirmed that Axton is indeed bisexual.
Originally, as part of the Bearer of Bad News quest the player would have to tell Tiny Tina of Roland's death. The event can be heard here.
For the Captain Scarlett DLC, originally the players would actually get to fight and kill Scarlett while Roscoe would be the new raid boss.
There was originally no plans to include a Buddy AI where that allied NPC is able to prioritize targets and attack them. The mission "Hunting the Firehawk", Lilith was to have a smaller role where she would Phase Blast once for her intro and Phase Blast to conclude the fight by killing any surviving bandits during the fight.
Originally Lilith's title card didn't say "aka The Firehawk". It was added after focus testers repeatedly complained that they were never able to find this "Firehawk" the mission was telling them to look for. Anthony Burch also rewrote one of Lilith's lines to clarify the point.
Before coming up with the DLC, "Tiny Tina's Assault on Dragon Keep", there was initially an idea where Handsome Jack had a spoiled nephew that lived on an island and would trap the Vault Hunters in a game that forced them to fight their way through a series of complex challenges he had designed. The idea later became a fantasy idea and Tiny Tina eventually became the dungeon master.
Among the various ideas for the new DLC was the theme of heavy metal rock and roll which was on top of the list. Ryan Heaton, the Lead Level Designer for the DLC, was glad that the team did not go in that direction.
An early storyline quest would have involved the player chasing down Fink (the Rat who hosts the Bandit Circle of Slaughter) and beating some answers out of him. This was too complicated to program, though it does explain why Fink hates the players so much.
Originally, Motor Momma of the Torgue DLC would have become a Raid Boss like Pyro Pete.
Handsome Jack would have originally played more of a Frienemy role to the heroes as a rival Vault Hunter who would help or hinder them whenever he sees fit. Eventually, the developers decide to make him purely a villain to make things clearer.