When a /clear command gets powerd, while your inventory is open and you are holding an item at your cursor, it will not be removed.

STEPS TO REPRODUCE:

1 Build the sircuit shown in the screenshots or download the map.

2 Put some items in your inventory

3 Press the button

4 Take one item on your cursor befor the signal reaches the CommandBlock

5 All items will be cleared out of your inventory, only that on the cursor will be left