If the player is blocking with the shield and has food selected in their hotbar, when they get hit and the shield loses durability, they will start eating, but will never stop (and hunger will not be regenerated). Additionally, their shield will still be up and block attacks.

Steps to reproduce:
-Put a shield in your off-hand slot and have food in your hotbar. Make sure you're hungry and don't have the food selected.
-Start blocking, then switch your selected slot to food.
-Let a mob hit you (Note: the mob should be powerful enough to break the shield)