Some cell phone mounts for bikes include a short tether as a backup in case the phone is dislodged. Whenever I mount a camera on my bike for videos I add a short tether in case the mount fails.

So far the only thing I've dropped in the chain was my water bottle when I fumbled it while trying a no-look replacement into the cage. Jammed between the chain and frame but didn't knock me off balance -- I didn't even need to stop. Chewed up the water bottle. I'm probably lucky it wasn't worse.