Math error?

Hi, I''m working on my own 3D engine but i''ve got an problem with calculating lightmaps. All the edges of my tri''s are black while they must be white. I saw that glexplorer had the same problem. A friend of mine told me he was told at school that this is a math problem with determining the distance of 2 vectors. Can anybody tell me if this is true, and how to overcome this problem? thanks!