Put the summary of the bug you're having here
Chunks randomly generating where they shouldn't be when you launch a 1.6.2 world in 1.5.2.

What I expected to happen was...:
Not really sure what I expected I got curious and decided to load into my creative world while running 1.5.2. I launch in 1.5.2 to play on a server.

What actually happened was...:
I loaded into the world and many chunks were totally messed up and most of my builds had been completely destroyed. The chunks were not reverted back to their original state, they were just completely random. i.e. there is an extreme hills chunk(which wasn't there before) in the middle of my castle which is in a plains biome.

I tried this on another world and the same thing happened

Steps to Reproduce:
1. Start a world in 1.6.2
2. Take note of chunks or build something etc
3. Launch the world in 1.5.2 and chunks are all screwed up!

Good thing I backed up my worlds around two weeks ago!
I am probably going to have to just delete the two worlds and start the last two weeks over on the backups.