The bug

In Creative mode, the hotbar sometimes becomes desynced if an item is moved immediately before and after switching to the inventory's Survival tab.

To reproduce

Note: Steps 1-4 must be performed in quick succession.

1. Open the Creative inventory (but not the Survival tab)
2. Place an item into your hotbar
3. Switch to the inventory's Survival tab
4. Move the item to a different slot
5. Exit and rejoin the world
   → You now have two of the item