When you use the last item in Croc’s Backpack, it actually remains in your inventory and you can keep using it infinitely. < when does it stop working? when dying, or leaving the level? >. For example, you can spam Gummi Jumps (which allows for the Infinite Jump glitch), or keep retrying a Clockwork Gobbo. Doesn’t seem to work for Keys though.
This glitch is not possible if you collect the Magic Eye Zoomers, because then Croc’s Backpack can never be empty.