Hello,

This is a strange glitch happening to me in SMP.

See Screenshot one:
You will see a Diamond Pickaxe - striking 3 times leads to its destruction.

After that, I opened my inventory and dragged the Diamond Pickaxe of screenshot two in the same slot resulting in a recreation of the previously broken item attached to the mouse-pointer (I still had 3 strikes left until it broke again).
I was able to repeat this as many times as I wanted, until I dragged another item than a Diamond Pickaxe into this slot.