What really happens, when the number of vertices are not 3n ?
I think a reasonable thing to do would be to throw an exception.
@CacheInTheTrash Yes, though when this gets implemented on a graphics card, it's hard for the GPU to throw an exception back to the CPU. I would guess the OpenGL spec says that the behavior is unspecified, meaning that different vendors can implement it in different ways, with no guarantee about what happens. In practice I suspect the vertices just get ignored (i.e., the GL driver tosses them out).