When an arrow hits the player from above while blocking with a shield, it can get stuck there causing the durability of the shield to go constantly down even if the player is not blocking anymore (as long as the shield is in his selected slot).
- Give yourself a shield
- Use this command and do not move after that: /tp ~ ~ ~ -90 0
- Switch to third person view (optional)
- Use this command: /summon Arrow ~0.1 ~7 ~
- Start blocking the shield while the arrow is falling
Note: The same thing happens for players in creative mode even without holding a shield