In certain (if not all) biomes, it appears there is something wrong with the generation. In certain places unusual patches of plants generate. What type of plant it is depends on what the biome has in most of. These patches seem to somehow be "hyper-generating" plants. [see images below for better description]

The biomes where these irregularities were discovered: "Swampland" biome and "Extreme Hills" biome (no extra criteria ie: M or +)

P.S. This type of generation is surprisingly cool! I personally think that, with a little tweaking, this bug could be turned into a cool new addition for world generation.