who's username was it?! Herobrines?! In that case, Jeb probably added something in that was not on the update list for a good joke. Otherwise, did you have your singleplayer world open to LAN? Then anyone in your local area with Minecraft (providing they have 1.3) can go into the multiplayer screen and their Minecraft (providing their internet is on) will look for any LAN open worlds in their local area. And they can go onto them to have multiplayer in a singleplayer world! That's why they added singleplayer chat and SOME commands like /gamemode /tp /seed and /xp. So if your world was jut open to LAN then you've got nothing to worry about! It's just another player coming onto your singleplayer world because you opened it up to LAN. If that's so, then the other player is obviously someone from your local area, because that's as far as LAN's signal will go. If you don't want anyone coming to your singleplayer world via LAN, then go into the pause menu (esc) and youll find next to the options button, there is a button that shows whether LAN is turned on or off. Turn it off if it's on.