So, just like the title says I'm encountering the Undergarden spawning in the Overworld. I've looked through the issues and I've seen similar ones w/ TF spawning in the overworld. I'm not sure if this is the same issue or not.

Here's some pics

The first picture spawned next to sludge pit, fen, and seasonal forest biomes.
The second picture spawned next to shield, grassland, and seasonal forest biomes.

I dumped the biome ID's w/ NEI and I'm not seeing any conflicts. Here's the dump log if you wanna take a look. http://pastebin.com/18mCuM5d

Any mods installed?

Yes, but I took them all out, save biomes o plenty, and re-created the world from scratch and it still happened.

Edit: Seems I may have spoke too soon. I hadn't tried deleting my config files last night, and I'm now unable to re-create the issue even with all mods installed and the same seed. I'm going to close the issue for now. I'll let you know if I ever encounter it again.