Once player exhausts the quantity of an items on an inventory slot (example eatting 10 carrots) and picks up an item that they are standing on (cobblestone for example) the new item becomes invisible in the inventory slot. As player cannot see the item, it cannot be moved or used. Player cannot change items with other items in inventory, it is totally unaccessible and blocks a slot from being used.

Closong and restarting the game reveals the item that was invisible and makes it accessible.

This bug was found l, and repeated in Hardcore mode