Rendering error during mesh deformation - vertex sticks to middle of viewport

I have an interesting visual crash that happens a fair amount in my geo-mod system. I have trouble repeating it as it tends to happen only when things are going crazy! Basically I'm creating lots of custom meshes and manipulating the verts. Every now and then I get this happen. (There are three examples here - it's not split screen). As I move the viewpoint, the black spike continues to end in the centre of the view. Has anyone seen this before? I'm wondering if maybe I've assigned a non-existent vert to a tri or something? alt text

Either there's a bug in your code or else your graphics card has some bad VRAM.

I've found the problem and for those who are interested, it's because somehow one of my vertices was being given position values of 'NaN'...