What I expected to happen was...:
I expected my Minecraft worlds to run.
What actually happened was...:
What actually happened was they kept crashing.
Steps to Reproduce:
1. Open the launcher;
2. Click either 'Singleplayer' OR 'Multiplayer';
3. Select a world and click 'Play';
4. It will not open and crash (occasionally it will open the world, but as soon as you move the mouse it crashes).