This bug also exists when looting chests of all kinds (chests, crates, etc.) You can be grouped or solo to reproduce this.
Start looting a chest and let the loot window open
Take any 1 item
Close the loot window, jump, or move (just close the window)
When you attempt to recover the items, you will get the "Already looted" error
The item you are trying to loot may display a different icon when you close and re-open the chest
I did not test all loot slots as in a situation where there are 2-3 pages of loot, but it appears to be any item you take will work. The loot type does not seem to matter, and the source of the loot window does not seem to matter (mobs, chests, crates and so on all work the same). I have NOT tested resource nodes yet but will edit this when I do. I was also able to bug my loot entirely by jumping and moving forward with the loot window open, but this is not a 100% occurrence (I had to delete WTF and WDB to fix the issue). Being feared away from a loot source will also cause this bug if you have taken an item from the chest. Shift+click (autoloot all items) normally as long as you have not bugged the chest first.
There is something else I wanted to add, but I will edit it in after more testing. This bug may also be in Mangos 1 and 2.