If a player is logged into Minecraft, then logs out for another player to log in, the second player's inventory and experience will change to that of the first player if he visits the same world.

Example: Player 1 hosts a LAN world from his PC, then player 2 joins it from his own PC. They play as usual and retain separate inventories. But, if player 2 logs onto player 1's PC and accesses the world, his inventory and experience become the same as player 1's. He cannot regain his original inventory or experience, even after logging out and rejoining via LAN.