when i get one of the snaphots when i get off of minecraft it goes back to 1.5.2

Put the summary of the bug you're having here
I had to re download the snapshot every time because it went back to 1.5.2
*What I expected to happen was...:*the snapshot would stay
Describe what you thought should happen here
it would stay the snapshot
*What actually happened was...:*the snapshot dissaperred and it went back to 1.5.2
Describe what happened here
it went back to 1.5.2
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...when I get off minecraft it happens
3. ...