Borderlands 2 Easter eggs

Find The Secret Claptrap...Again
When visiting Arid Nexus-Badlands, formerly Fyrestone, the hidden Claptrap from the original game can be found for 5 Badass Rank (video).

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 a Minecraft easter egg. Inside are several Creepers and a Badass Creeper (who can drop the Longbow sniper rifle or the Sunny Blockhead shotgun), some of whom will drop Minecraft themed skins and heads. There are also Minecraft blocks, including dirt, stone, coal ore, gold ore, and a new block unique to Borderlands, Eridium ore. When broken, coal ore will drop ammo, gold ore will drop money, and Eridium ore will randomly drop either Eridium or a Minecraft-themed class customization.

Double Rainbow
On an isolated cliff in the The Highlands - 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 What Does It Mean? achievement (video).

Tannis Rides Again
In Sawtooth Cauldron, in the bay near Main Street Reservoir, you can shoot five barrels in succession to reveal Tannis once again riding a fish, set to dramatic music (video).

Roland's Facebook
In Sanctuary's Crimson Raiders HQ there are several monitors, once of which is Roland's Facebook page, revealing he is in a relationship with Lilith. Her messages read:


 * "Hey, i love you call me"


 * "was just thinking about you again"
 * "was just thinking about you :)"

Roland's occupation is listed as Hired Gun and he lists the year he graduated as 2854. Mordecai and Brick are visible on his friend's list.

There is also a monitor with a picture of an overweight blonde in a bikini.

In the Firehawk base one can also find a 'blue screen of death' with the message:

"So, hate to be the bearer of bad news, but your computer is pretty hacked. Maybe if you'd downloaded less of that illegal Truxican porn, your machine wouldn't be all jacked up. You should probably be embarrassed. If this is the first time you're seeing this, that's only because no one else has caught you yet. Tell you what's gonna happen here...you're gonna put a new graphics card in me and scrape 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 laying down?"

QR Codes
There are three unique QR codes to find in the game. In Sanctuary, check the top left most body freezer in Zed's shop (video). One can also be found in a pizza box over at Moxxi's, and on the invitations to [http://borderlands.wikia.com/wiki/Claptrap%27s_Birthday_Bash! Claptrap's birthday party] and various postcards throughout Sanctuary.

Terramorphous
If the vault hunter is in the Bunker and looks towards Terramorphous Peak for a few minutes, Terramorphous can be seen on the lake (video).

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.

Dark Souls
On the eastern part of Caustic Caverns there is a small island with a lone tree. 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 shield 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 in the game Dark Souls. You can also spot a soul of a knight sitting by the camp. Reaching the island also unlocks the Badass Challenge "I Bet I Can Make It," a hidden challenge in the Caustic Caverns.