When playing on realms, every-so-often, items in your hotbar will either: 1.disappear into your inventory
2. 'Uncraft' themselves (e.g. Diamond sword becomes 2 diamonds and a stick, in your inventory)
3. Completely vanish - not in hotbar or inventory.

This is a pain, in a final of a survivalgames deathmatch, when your only diamond sword vanishes - many matches have been lost this way.