The bug

Normally the /clear command returns how many items it has cleared from a person's inventory. But /clear does not count the items which were in the player's inventory crafting grid.

To reproduce

1. Place a command block and put the command /clear @p into the command block.
2. Place a button which activates the command block after some time (eg by using repeaters)
3. Press the button
4. Open your inventory and put an item into your crafting grid.
5. Wait for the clear command to run.
6. Open the command block and see "Previous Output" says no items were found even though it found and cleared your items.