What I expect to happen is that when you leave save and quit the game, I should be immediately able to enter the world again, right where I left off.

What actually happened is that the game was stuck on the "Loading World" screen, and I had to restart Minecraft for the world to even open. When I did, there was a rollback to when the world stopped responding.

STEPS TO REPRODUCE:
1. Play in your world Imine was made in Beta 1.7) for some time, the bug
activates randomly.
2. Check if you are able to pick up items, use redstone, or type commands.
3. If 2 is true, then save and quit.
4. Try to enter the world again.
5. the game should be stuck on the loading screen, and you should have a
world rollback to the time where the world stopped responding.

EDIT I just tried this with a new map I just made, it happens there too.