While in the nether. I had my diamond sword in slot 1, and my iron pickaxe in slot 3. I did some mining, switched to sword to fight. Then trying to retreat, I had used the sword to mine some netherrack or nether quartz to get away. Then suddenly my sword in slot 1 became an iron pickaxe. This happened twice on different occasions, both in the nether. And I still had my iron pickaxe in slot 3. So I ended up with two iron pickaxes and no diamond swords.

FYI
I use city texture pack.
This happened on a realm that I own.