Basically, when I made a custom generation world in 1.8 pre-1 and switched to 1.7.10, the game instantly closed. When I tried again, the game tried to load and crashed again.

What I expected to happen was...:
I would see my custom generation, and where the default 1.7.10 generation took over, I would see a sudden change in world generation.

What actually happened was...:
The game crashed.

Steps to Reproduce:
1. Make a custom generation world in 1.8 pre-1.
2. Explore around a bit, to load in some chunks.
3. Close the game and switch to 1.7.10
4. Open the custom world.
The game will crash upon loading.

Note: I cannot join any world after multiple attempts to restart the game. Don't replicate if at all possible.