Last night I picked up a small anvil which had a branding iron sitting on it and logged out for the night. Then today I saw the branding iron on the ground where the anvil had been, and was confused because I remembered picking it up. But I couldn't pick it up, and it also appeared in my inventory in the small anvil container slot. So it looked as though there were two of them.
The one on the ground couldn't be picked up, but I could interact with and examine it as normal. It had the options to drop or place as if it was actually in my inventory, but it appeared on the ground. It was possible to interact with it though. I can use drop or place on either one, and then the "extra" one disappears and it's all back to normal. I tested with a few random items and couldn't get it to happen, but it happens with the branding iron every time.
Steps to reproduce:
Place a branding iron on a small anvil, and pick up the anvil.
Relog, wait a few seconds and the duplicate item appears on the ground at your feet, but is also still in the anvil container slot in your inventory.