It happens sometimes that a slot (or it could be the item in that slot) glitches, causing that item to be stuck there, and cannot be used or moved inside the inventory. If I tap that item, and then I try to swap it with another item in a different slot by tapping another slot, the glitched item will swap place with the normal item for a small fraction of a second, and then it will teleport back to the original glitched spot (and of course, the normal item will be back to its original slot). If the glitched slot/item is in my hotbar, then I can't use it: if it is a block, and I try to place it, I will see the block placed it for a fraction of a second, and then it will disappear and go back in my inventory. If it is a consumable, I can see that I am using it, but it won't have any effect, as it will always undo the action (if the glitched slot/item is a 30 apple, and I eat one, I can see the number go to 29 for a millisecond, and then to 30 again, and I will still be hungry. Same thing with blocks)