I was mining in my singleplayer world. Strip mining, straight path. Then I run into a 'wall' of bedrock that is blocking my path. Strange... Right?
Well, I turn around to leave the mine. I have enough ore..
There is bedrock there, too.
Keep in mind cheats are OFF and I have not touched the LAN Button.

Minecraft starts to become very laggy, then the lag stops and it looks like something from the Super Secret Settings.. I haven nausea too. The thing is I am in 1.9 so the Super Secret Settings shouldn't be there. I start to mine in the other direction. where there is no bedrock. I turn around and take a screenshot the chat says Error iI. I don't know what it means.. I keep mining and suddenly, my hearts are gone and it says in red text (where the health is supposed to be) -11

At this point I'm freaked out. I know you can get negative health but only with cmd blocks and certain glitches. So its impossible. I press tab and there are TWO PLAYERS on it. THIS IS A SINGLEPLAYER WORLD WITH NO LAN, MODS OR COMMAND BLOCKS! The username is called Error 220..
Heres were it gets even more creepier. His skin (which is shown on the scoreboard) is nil. I turn on lan and put on cheats and I do /tp ZyIock Error 220
It says No. in chat.

Suddenly, I die and it says what it would when you died in hardcore mode. It wouldn't let me respawn, but I can spectate. I click spectate and I find Error 220. He is sitting in the middle of a field, where the spawn was. But it was only his head, the rest of the body was invisible.

I did some research and Error 220 means this file is checked out or locked for editing by another user.

this file is checked out or locked for editing by another user.

checked out or locked for editing by another user.

locked for editing by another user.