If you drop a fully charged crossbow, the crossbow will appear to have no arrows when it's dropped but if you pick it up again it still has an arrow.

(I haven't tested all versions)

How to reproduce:

1. Get a crossbow and fully charge it so there's an arrow in it.

2. Drop the crossbow on the floor.

3. Realise it doesn't have an arrow.

4. Pick the crossbow back up.

5. Notice it now has an arrow again.