This just means that the surface of one of your blocks is too big for shaded mode. There are a few causes:

1. (obvious) One of your blocks covers a very large area. Either split it up into multiple blocks, use flat shaded mode for that surface in question, or use models.
2. You have too many surfaces right next to the lightmap space limit. Use fewer surfaces or use models for the smaller details.
3. You have a fairly small block but with a tiny texture scale (such as a 0.1 scale). Never have a texture scale below 0.25 for best results.