I defined some glossy planes by specifying the four vertices and vertex order for each. I rendered them in Cycles under an outdoor HDRI lighting. Strangely, some of the planes split into two triangles rendered in very different colors.
At first I thought it was due to surface normals, so I made sure the surface normals all point to the camera. However, the problem persists.
Why is it so, and how may I fix this? Many thanks!