Put the summary of the bug you're having here

What I expected to happen was...:
Enter my single player world and play like normal.

What actually happened was...:
I opened up a single player world in 1.7, after switching to a 1.7.2 profile from a release 1.8 file. I did this because I wanted to do some things before updating that world in particular. When it opened, My inventory and everything in all of my chests was gone. Any additional data (even my achievements) were gone. This was not my MAIN world, but now I'm afraid to open any other old world because everything might get erased.

Steps to Reproduce:
1. I'm not quite sure but it had something to do with reopening old worlds after switching between 1.7.2 and 1.8. Not sure how often it happens.