If you attack someone with a sword and the damage is canceled by a plugin (GriefPrevention in non PvP area, or McMMO from friendly attacks on party members, etc etc) then the client gets out of sync with the weapon durability.

 

Continuing to use the item while it gets canceled will eventually show the item getting destroyed on the client. Once destroyed you can click on the empty slot where the item was and the item comes back with the proper durability again.

 

How to replicate:

 

Scenario 1) Attack someone in a non-pvp GriefPrevention claim until your weapon destroys. Click on empty slot where weapon was.

 

Scenario 2) Attack an mcMMO party member (with friendly damage disabled) until your weapon destroys. Click on empty slot where weapon was.

 

Possibly happens with all plugins that cancels the damage event for any reason (WorldGuard, Towny, Factions, PlotSquared, etc etc).