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.