When joining a world there is a chance for 2nd user to lose their character (everything from inventory and gender, even xp, levels) tends to happen very rarely and if player leaves game then rejoins a few minutes later they will regain their character and all values linked to the player entity such as location inventory xp and ender chest but if a item or xp is gained the old player will be lost when exiting the game (happens infrequently and is definitely scary when leaving with valuables early game.