I'm trying to draw terrain from a heightmap by using a VertexController applied to a plane. I get abnormal behaviour when applying height values to the vertices on the starting edge of a plane.

Screenshots:
Picture 1 (what I want to happen):http://dy.fi/7m0
Picture 2 (the problem):http://dy.fi/3ci

The plane is modified using nearly identical code, by setting a single row of the heightmap to a contain the same value in all of its columns. The only difference is on which row of the heightmap the same height value is. The artifacts in picture 2 only appear when applying height values to the first [quads*2] vertices of the plane. Basically in a nutshell I need the heightmap to apply consistently to all the vertices in the plane.
Sorry if I'm being too brief, but I lost my previous message when the forum logged me out because of the default timeout.