There is a small tent located in the Highlands. Entering the tent triggers a double rainbow to appear on the horizon, with a sound clip from either Handsome Jack or Claptrap imitating the Double Rainbow Youtube video.

The targets of the mission "Splinter Group" are four mutated raiders named Mick, Lee, Ralph, and Dan. This is a play off of the Teenage Mutant Ninja Turtles named Michelangelo, Leonardo, Raphael, and Donatello. The other similarities include their hideout being in the sewers, their name "Splinter Group" after the rat master "Splinter", their surfer-like language ("dude", "brah") and their craving for pizza used to lure them out.

There are a number of QR codes scattered around Pandora. Scanning these codes using a QR code scanner will result in different messages.

Pizza Boxes: "Wise man say, forgiveness is divine but never pay full price for late pizza. -Captain Picard."
Possibly a reference to 4chan, where many pictures of famous characters are misquoted. In this case, a Michelangelo quote from Teenage Mutant Ninja Turtles and Captain Picard from Star Trek: The Next Generation.

In Zed's Pharmacy: "Thank you so much for playing our game, now I'm rich with love AND money! !!Nick Wilson!! Ditto!!!"
A reference to the artist Nicholas W., an artist at Gearbox.

Various Places: "NICK WILSON/ /HO OH"
Same as the previous message, a reference to Nicholas Wilson.

In Borderlands 2, There's a character named Michael Mamaril that will give the player free (and usually rare) loot. This character is based on a real-life person who died at the age of 22. His friend wrote to Gearbox, the game's developers, asking them to do a eulogy using the voice of the fan-favorite character "Claptrap". Gearbox not only fulfilled his request, but also added Michael as a character in Borderlands 2, so he could live on forever in Borderlands.

According to "Inside the Vault: The Art & Design of Borderlands 2" that comes with the special edition of Borderlands 2, when the boss "BNK-3R" makes a noise, it's actually the game's sound designer Joshua Davidson yelling about tea.