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Fake map item by clearing filled_map from inventory

Shift-clicking X button in Creative menu can create ghost items

Inventory can get desynced when shift-clicking the Destroy Item slot

Shift clicking items into hopper can create ghost items, and rarely duplicate them

Advancement reward function creates ghost item

Allays and villagers create ghost items when they take items from a stack and cannot fit the whole stack in their inventory

Placing a block immediately after picking it up produces a ghost item