Chek this out https://i.imgur.com/aqpprcM.jpg . Dark Forest Center in the overworld. Using Biomes O Plenty and BoP worldgenerator. No biomes id conflict according to this http://paste.ee/p/kCzt5 . Guess it's either TF or BoP, but I dunno. Posted in both.

As that seems to be "Dark Forest Center" generating as a sub-biome of the "Bayou" biome and their IDs match the pattern (they are 128 apart from one another), this might be just this issue: #334

It should be fixed in builds .989 and above though.
So if you got a version number below that, just update your BiomesOPlenty.
If you are already using a more recent version, than this could be something new, so your version number makes the difference here. Which I write to remind you that you could just have mentioned which version of BOP you use in the first place. ;-)

Always using latest version, in this case .993 Been about half a years since i last played MC, don't remember what I'm supposed to post or not with issues. Appologies.