Backstory

Since 1.13.1, tools and armor with 1 durability break when used. Before that, they could be used one extra time because 0 durability was a thing.

The bug

Mobs can still drop tools and armor with 0 durability.

How to reproduce

1. Use a zombie/skeleton/any-mob-that-can-have-equipment spawn egg a lot of times on the ground.
2. Start killing them until you get a piece of equipment that has 0 durability.
3. If you don't get such an item, do the same thing all over again.