When a redstone signal is given to a hopper originating from a redstone comparator detecting its item, the dropper will not fire.
To Replicate:
1. Place a dropper with any item inside.
2. Place a comparator to measure its capacity.
3. Connect the comparator to a redstone repeater, and place a redstone line to a sticky piston in a positon so that the block it pushes will be next to the dropper.
4. Place a redstone block on the piston.
5. Place a button on the dropper.
You should see that it does not fire, despite that it should because of the signal powering it.
(I know well that this shouldn't work with providing a temporary signal because the redstone block powers the comparator indefinitely, but the dropper should still fire!)