*Notice: This bug is quite rare, if you cannot reproduce it is a typical phenomenon.
Break a flat world first.(a.k.a. suddenly log out a minecraft flat world until it becomes infinite)
Break it a few more times after the flat world turns infinite.
At a point, you are only able to move a few blocks away from where you ended up last time you played. Walking further(technically out of the current chunk) will break minecraft and crash instantly.
This has broke two of my worlds, causing these worlds unable to play, and I am waiting a bug fixing response for a long time.
If this description is not clear, please edit this.