Setting a command block to clear a specific item from a player (I used "clear @a 281" to test this) and attaching that command block to a clock of any speed means that if you are in your inventory and in creative and you are moving an item around the inventory with your mouse as the command block is powered, the item held by the mouse pointer vanishes. This happens whether the specified item to clear is in your inventory or not. In survival and in Minecraft 1.5 creative this didn't happen.
Linked issues
Comments 3
Step to reproduce:
Create a command block with "clear @a 137"
Create a clock that power the command block
Take any item
-> This is a bug, you can click on any item, it will disappear even if it's not a command block (id: 137)
Duplicate of MC-18627, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.
It was a bug that you could do that in 1.5. So I think this is intentional.