I’m really sorry that I don’t remember how this happend or how to reproduce it, but I’m gonna report it anyway.
I had this stack of foliage in my inventory that i was not able to move. (Picked up from the ground after gathering.)
Every attempt to move it out of my inventory created a copy of that stack. These stacks could be split, merged and even put back in my inventory.
My first thought was: OMG dupe glitch … I’m going to be rich! … and banned.
But not really. The bug is only visual / client side. Relogging removes all the copies.
I finally found a way to “unbug” the stack by swapping places with an other item in the inventory.
After swapping, the stack behaved normal again.
Now that i think about it, this happened to me before with items dropped by another character.
Back then I though I’ve lost the items because it looked like they disappeared from my storage.
It was not a big deal, because it was just some stuff from the woods.