There are lots of bugs and easter eggs in Minecraft, and I know that there are some you don't know. For example, there are four giants in Minecraft. The first is well known. The giant zombie. Then, there are giant slimes. Those aren't really new either. But, there is something AWESOME. Giant horse's heads and withers. Not just talking about a normal horse and wither. I'm talking about giant higher then height limit withers. GIGANTIC. And giant horse heads, which are about as big as a five blocks. Most of these are bugs, though.
Giant: Just a mob that has been in Minecraft since beta, but just doesn't spawn naturally.
Giant Wither: This is actually a bug. You know when a wither spawns? He flashes, while he regenerates. Eventually, he blows up. Between the time he regenerates and the time he explodes, he gets little bigger. Using the /summon WitherBoss command, and setting his invincibility to some crazy number, he spawns as a HUGE wither. It glitches though, and doesn't move. It's about 500 blocks high....
Giant Horse head: It is common knowledge that a horse grows bigger when it has been spawned as a baby. But when setting the age to something like -25000, it glitches horribly. The head is high above the body, and the legs are everywhere. It's almost funny....
Naming Something Dinnerbone: If you have a nametag, and use it to name an animal dinnerbone, it will flip upside down. You can also name it Grumm for the same effect. They are both developers of Minecraft, and always seen upside down.
Naming Sheep jeb_: This will cause the sheep to change colors randomly, though when sheared it still drops the same kind of wool as it had before. Jeb_ is currently the lead developer of Mojang.
Changing Language To Pirate Speak: There is a language in the languages option called Pirate Speak. It changes the text to pirate-like speech, like Arr, and timbers for wood. It's quite hilarious.
Holloween Easter Egg: During the week of Holloween, monsters will spawn with jack-o-lanterns on their heads!
Christmas Easter Egg: It's not exactly an easter egg, but during Christmas time, all of the chest textures will change to presents. There is also a skis texture which I found, but it is unused.
Spider Easter Egg: While in the course of modding, I was looking in the Java code for the spider. I saw that it had a chance to spawn with a random potion effect. I think it may be about a 5% chance. I tested it out, and sure enough it worked!
Zombie Pigman Texture: If you open the zombie pigman texture, you will see the text 'THX XAPHOBIA'. This was to give credit to the creator of the texture.
Never Displayed Splash: A Minecraft splash is the text next to the Minecraft logo in the main start screen. There is a file called splashes.txt in Minecraft.jar, and one of the splashes is "This message will never appear on the splash screen, isn't that weird?".
Minceraft: There is a 0.01% chance that the Minecraft logo will be Minceraft on the main menu screen.
Notch's Apple: If Notch, the original creator of Minecraft, dies ingame he will drop an apple along with his inventory.
Dinnerbone's Death: A Famous Youtuber, Docm77, killed Dinnerbone in the first episode of Mindcrack Ultra Hardcore season 11. Dinnerbone added "What's Up, Doc?" to the splashes.txt, and had a bug fix in the next Minecraft update that said "Removed mob Docm77, not working properly".
Disc 11: There are a bunch of Minecraft music discs, and Disc 11 has a little easter egg. When seen with a wave viewer (one of those things that let you see the waves of sound going up and down), you can see a Steve head.
Herobrine: Everyone knows Herobrine. He is in a few easter eggs. In most Minecraft updates, it says '-Removed Herobrine'. He does not exist.....