After closing and reopening a world save, any recent changes made to the world are reverted, but the player's inventory is unaffected.

For example, if a block is placed, then the world is closed and reopened, the block will sometimes disappear completely. This is the same for any large structures built.
Also, if the player takes an item from a chest and places it into his or her inventory, closes and then reopens the world, the item will still be in the players inventory but it will also still be in the chest (a double up of items)
This also affects mobs: if any number of mobs are killed, and then the world is closed and then reopened, the mob/s will still be there as if it were never killed, yet the mob drops will still be in the player's inventory.

Edit: This issue happens sporadically, but not all the time.