Wikia

"Blue Screen of Death"

In the Firehawk's lair, one can find a "blue screen of death" with the message:

"********STOP****(unreadable numbers)

Error 1068:230

So... hate to be the bearer of bad news, but your system's pretty hosed. Maybe if you'd downloaded less of that illegal Truxican porn, your machine wouldn't be all jacked up. You should probably be embarrassed.

2MUCH_PRON_MXYZ.SYS

If this is the first time you're seeing this, then it's only because no one else has caught you yet. Tell you what's gonna happen here... you're going to put a new graphisc (sic) card in me and scrap (sic) that gunk off my keyboard, and then maybe your search history won't accidentally be mailed to your grandma - you pickin up what I'm layin down?

********STOP****PLEASE GOD STOP******"

Dark Souls

In the eastern part of Caustic Caverns, there is a small island. This island can be spotted from the mission "Safe and Sound" just behind the giant crystalisk, Blue. The island is separated from the mainland by a caustic pool and reaching it requires an adequate amount of shields and health. The island consists of a lone tree and a camp with a lone figure named Solitaire, who is a reference to Solaire of Astora from the video game Dark Souls. Near the camp, the soul of a knight can be seen sitting.

Upon reaching the island, the player will also unlock the challenge "I Bet I Can Make It," a hidden challenge specifically for this Easter egg.

"Double Rainbow"

On an isolated cliff in the The Highlands at Hyperion Bridge, there is a small camping area with a tent. Reaching this area will cause the "Double Rainbow" Easter egg to appear, granting the player the "What Does It Mean?" achievement. A video of the egg can be seen here.

Find the Secret Claptrap... Again

When visiting Arid Nexus - Badlands, formerly Fyrestone, the hidden Claptrap from Borderlands can be found for 5 Badass Rank. A video of the egg can be seen here.

Johnny Waffles

Johnny Waffles in Sanctuary is considered an Easter egg, as the character was added as a reward for an actual Easter egg hunt at PAX East 2011.

The Lord of the Rings

When the location Eridium Blight is found during True Vault Hunter Mode, Claptrap's Place will have a gear in the fireplace. Picking up the gear starts a series of events beginning with a walk to the Eridium Blight and ascent of the volcano there. Geary, an enemy psycho based on a character from The Lord of the Rings, Gollum, will attack and then jump into the lava. Presently, rakk will drop several chests drop from the sky with loot. This is all a great reference to the famous movie The Lord of the Rings: The Return of the King.

Minecraft

In the northwestern area of the Caustic Caverns, at the top and to the right of the train tracks, there is a hidden cave containing an easter egg referencing Minecraft, a popular independent video game. Inside the cave are several Creepers and a Badass Creeper (that can drop the Longbow sniper rifle or the Blockhead shotgun), all of which are references to famous enemies in Minecraft. The creepers will sometimes even drop Minecraft-themed skins and heads. There are also Minecraft blocks in the cave, including dirt, stone, coal ore, gold ore, and a block unique to Borderlands, Eridium ore. When broken, coal ore will drop ammunition, gold ore will drop money, and Eridium ore will randomly drop either Eridium or a Minecraft-themed class customization.

QR Codes

There are three unique QR codes to find in the game, all of which are found in Sanctuary. In Dr. Zed's clinic, one is in the top, left-most body freezer. Another is found in a pizza box inside Moxxxi's, and on the invitations to Claptrap's Birthday Bash! and various postcards throughout Sanctuary.

Roland's Facebook

In both the Crimson Raiders HQ and the Firehawk's Lair, there are several monitors, once of which contains Roland's Facebook profile, revealing he was in a relationship with Lilith at some point in time. Her three messages to him read:

"Hey, i love you call me"
"was just thinking about you again"
"was just thinking about you :)"

Roland lists his occupation as "Hired Gun" and the year he graduated high school as 2854. Mordecai and Brick are visible on his Facebook friend's list.

Tannis Rides Again

In Sawtooth Cauldron's bay near Main Street Reservoir, one can shoot five barrels in succession to reveal Tannis once again riding a fish, set to dramatic music. A video of the egg can be seen here.

Bioshock

in Magnys Lighthouse near the elevator used to ascend to the light house one can find a unique Anchorman. Mr. Bubbles has a chance to spawn with a unique midget called Lil' Sis. these are a direct reference to the the game Bioshoock By 2k.