I've noticed that in the new snapshots and pre-releases that when I break a tool in the nether then go through the portal I spawn in the overworld with the tool back on my hotbar with 0 durability. the broken item can then be repaired on an anvil.

I've also had a tool broken in the nether pop back into existance when another tool was put in its place on the hotbar. it appeared as though the tool was just dropped from the inventory.

as of yet I have not figured out exactly what conditions result in returning tools that have been worn out in the nether. still looking though, I will post updates on the issue as I find them.