As mentioned, the uppermost right corner inventory slot gets item stuck in it only when in the inventory UI though. Items can't be taken out, swapped/replaced with other items and items can't be placed into the slot. However in the chest UI, furnace UI or crafting UI you can move items in and out of that slot. The mouse cursor also gets stuck when trying to move it over that slot and can't be moved through the square but gets stuck on the border.