The bug

The can load a crossbow with air if you drop an arrow while charging the crossbow.

How to reproduce

1. Put crossbow in off hand
2. Put arrow in main hand
3. Hold right click to start charging crossbow
4. Press q to drop arrow
5. Wait until crossbow is fully charged
6. Release right click
   → Crossbow is now loaded with air projectile

See 2019-05-05 02-40-21.mp4 if you have trouble reproducing