Plastic bags might be another one to mention.
Bilbo loves the things (carrier bags, freezer bags, polypockets etc) and some time ago now I was clearing the litter tray out and found that more than half of the large poo was carrier bag, it had never crossed my mind until then that a cat might eat a carrier bag. I dread to think what would have happened if it hadn't passed through naturally. Now whenever he's inside I make sure to 'debag' the house first.