Cow Challenge:
(Direct copy from Minecraft forums, based off of two other ridiculous challenges someone else made)
In this challenge you must collect a full inventory of leather within 5 minutes of having a wood sword.
You have 2 minutes to find a diamond sword, and then you must get a wood sword also, but you can only smack the cows with a golden apple, which you have to find in a dungeon after 3 seconds, or get an apple after 1 second and also get 72 gold in 10 seconds, and craft a golden apple.
Then, once you have a full inventory of leather, you still have more to do.
Then, you must find a floating island within 1 minute, then you have to get on it and expand using any blocks in your inventory, then get 1006 leather up there, then you can come down.
BUT! The final step is to make 950 cakes within 10 minutes, to allow time for crafting them. You only have 5 seconds to get 2850 milk buckets, 950 eggs, 2850 wheat, and 1900 sugar from sugar cane.