Items in Furnace Randomly Disappear?!

When I went mining on a Multiplayer world, I put 45 Coal and 9 Iron Ore into a Furnace. And I left the Furnace to get more Ore, but I waited until I could see the fire inside the Furnace. The fire never showed up and i waited 3 seconds after to see what would happed. So I re-entered the Furnace and I saw everything there for 2 seconds, then everything vanished into thin air?! Please help and respond to this. (:<