Put the summary of the bug you're having here

What I expected to happen was...:
The world to spawn properly.

What actually happened was...:
The world around me spawned normally for maybe a hundred blocks or so, then there was a 25-50 block span of land that flickered and changed maybe ten times per second, and more when I moved around. Also, the animals spawned as random pieces of terrain, such as trees (without textures) or pieces of the inside of caves. When a squid would spawn, it would resemble a weather mod tornado, with random sets of blacks spinning around in a huge circle. I have pictures of all of this, though no video. It's very disconcerting.

Steps to Reproduce:
1. Open minecraft and create a new world in creative mode.
2. Select World Type: Customized, then click Customize.
3. Set Lava Lake Rarity to 60, then the Biome to Plains. Then, go to the next page. Set both diamonds and lapiz to spawn size 10 and spawn tries 10. Then, start the world.

I tried repeating this twice, and both times came up with the same phenomenon. Please let me know if this is an actual but, or just my computer (even though it's never done anything like this before)