When viewing a grass block from a very oblique angle (nearly parallel with the face), the grass part of the block turns black. The error fixes itself when you approach.

This only seems to happen in single-biome worlds (possibly something to do with custom biomes? - I saw it when I opened a single-biome world with Lush Caves.) If you look closely at the sides of many of the grass blocks in this image you can see that they have the error i mentioned above.