When you have a pickaxe with unbreaking enchantment and 0 durability (found by pressing F3 + h), after using the pickaxe a couple times will cause the breaking animation, and the pickaxe will disappear from your inventory (normal behavior). Upon quitting and reloading the game, however, sometimes the pickaxe is back in your inventory (abnormal behavior). Also, if you try to put something else where the pickaxe was, upon using this item sometimes it will also disappear until you reload the game (abnormal behavior).
This seems to happen fairly often.
See the attached video for an example.
What was expected:
When the pickaxe reaches 0 durability, it should not break or disappear for several more uses (because of the unbreaking enchantment); once it breaks it should not reappear.
What actually happened:
The pickaxe broke after a couple uses at 0 durability, and sometimes reappeared upon reloading. Items put into the spot where the pickaxe was sometimes disappear.
Steps to Reproduce:
1. Use command /give playername 257 1 250
2. Enchant iron pick: /enchant playername 34
3. In survival, use the pickaxe until it breaks.
4. If the bug took place, items will not be able to be picked up into that spot.
5. If the bug took place, Right-clicking on the item slot will make it reappear.
6. Save and Quit, the reload the world; if the bug took place the pickaxe will reappear.