Unexpected error

Put the summary of the bug you're having here

What I expected to happen was...:
To play a world for 1.7.2 with 1.7.2 and not using 1.8... I had downloaded the 1.8 for a try out but am still using 1.7.2 until the servers out there do there updates.

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

Steps to Reproduce:
1. Run the Launcher 1.5.3
2. Press the Play button.
3. Press the Singleplayer button.
4. Selected a world and then pressed the Play Selected World button.
5. Crashs! This have been happening ever since the 1.8 update.