The Problem With Planar Faces

Some faces created by Extruder with Quad faces may not be flat. This is due to the fact that quad extruded faces have 4 vertices. It is not guaranteed that these 4 vertices will be on the same plane. To check this create a 2x2 TGA file like the following and extrude it

White White
White Grey
|
V

If you look at the object in wireframe you can see that two flat triangular faces should have been created instead of one 4 vertex face with a big fold in it. To overcome this use triangular or adaptive faces.