How to reproduce:
1. Create a world.
2. Go in the world.
3. Let the chunks render a bit.
4. Leave the world.
5. At the menu, double-click your world. DON'T MOVE YOUR MOUSE!
6. Spam your left-click button as fast as you can!
7. The loading-screen glitches while you click at the spot where your world is in the menu.
8. When your world loads up, you will sometimes sink through the blocks. (look at the image!)