April Fools 2011
The Locked Chest Easter Egg was a naturally generated block that was used for the 2011 edition of the annual April Fools Joke by Notch. If the player attempted to open the chest, they would get a message stating that a key from the Steve Co. was needed.
Clicking "Proceed to Checkout" would trigger a screamer followed by a dinosaur from Jurassic Park to begin to fly across the screen.
Entering items at a value of over $10,000 would send a pop-up message asking if you were prone to seizures. If the player would to say yes, the screen would start to flash.
If one assessed the script to the Dinosaur Easter egg, a reference to the Konami Code would appear.
On October 31, 2012, all Zombies, Skeletons, and various other hostile mobs had their standard heads replaced with Pumpkins or Jack-o'-Lanterns. Killing them would have the block on their head drop.
link to article here.