When a player disconnects, their inventory is put into a chest for other players to plunder. When the player reconnects, whats left will be put back into their inventory. If there was a block there (like a pressure plate), the block will be placed into an otherwise locked Itemslot in the Inventory. When the player joins again and the chest hasn't been destroyed and replaced with another block, the block will be placed back. If the spot is occupied by another block, it will simply drop as an item.

Big 2.0 Update!


Take a look at the inventory:

The compass is in the offhand slot, other slots in the first row are partially or completely locked. 2nd row is the player's hotbar, the other 3 are the rest of the inventory.
Try it out!