In Snapshot 14w25a (Unknown if happening in previous versions), there is a block duplication glitch involving swords, blocks, and the creative inventory.

Steps to Reproduce Issue:
1. Go on the Creative gamemode.

2. Get any block, and any sword and put them in Hot-bar.
(I used a Diamond Sword and Diamond Block, although it did work with other swords and blocks)

3. Open inventory, and switch the positions of the sword and block by pressing numbers on keyboard.
Ex. Sword is in slot 1, Block is in slot 2. Press 2 while hovering on Sword in slot one, and now they swap positions. This part is normal

4. Now exit inventory and select the slot with the sword. (Press number of slot on keyboard, I'm not sure if scrolling to it affects it)

5. You will notice that the sword can break objects which it is not supposed to, and if you right click with it, it becomes that block along with placing one on the ground.

The block is a literal duplicated block. Stack it, drop it, whatever you want, but you now have 2 of the same blocks in your inventory along with one on the ground where you placed it.