- Terrain at spawn and around it was simply reset and replaced with another biome. Unsure if it was caused during 1.6.2 when the map was first created but it did destroy half my spawn village, some supply roads for horses, and regenerated loads of terrain. Not a simple terrain reset and regeneration, it goes quite a long way in both east and west.
(Possible helpful information)
It is an SMP server hosted by a friend. I once renamed it from "World" to "Vanfer" to give it a proper name for rendering maps. It also transitioned from 1.6.2 into 1.6.4 with the terrain around spawn simply regenerating. Nothing else really regenerated. Only new chunks spawned some odd new chunks on the "Vanfer" name, not the world or so I'm assuming.
I'm pretty sure that leaving the world name as is or renaming shouldn't have done anything that drastic, nor should it have regenerated existing terrain. My guess is something happened in 1.6.2 or 1.6.4 is to blame.
(I did a search but didn't find anything exactly as this one. Maybe I'm blind or maybe one wasn't even added in the same version as my issue.)
First image is an untouched and new spawn settlement with the rest being terrain being regenerated. Half of it was ocean and the other half a frozen wasteland.