This is more apparent in texturepacks than in default, but when viewing grass blocks on the edge of a biome, the transition is reversed. Say we have a jungle on the left, and extreme hills on the right. The grass should fade from a bright green on the left, to a blue-green on the right, however, this is backwards, which makes transitions between biomes less smooth. The parts that should be jungle, are extreme hills, and vis versa.

Sorry if this isn't very clear, it's rather difficult to explain. Perhaps the screenshot will help clear up any confusion.