Z-fighting
From DeleDWiki
Z-fighting is an effect which occurs when two polygons are placed onto each other. You will see it as a bad flicker, where fragments of both polygons are rendered at the same time.
The reason for z-fighting
DeleD uses OpenGL to display the geometry. OpenGL internally works with a z-buffer, which stores the depth information of each pixel to render. Now, if you have two polygons at the same plane and position, the z-buffer won't know, which polygon it should render on that pixel, so the choice is made fairly arbitrary. And because of that, you will see polygon 1 at some places and polygon 2 at others in that area.
The solution
You should keep your models clean from polygons with the same position. Move the polygons slightly forward, to avoid z-fighting. If you have problems with z-fighting in your engine, you can setup the depth buffer so that only fragments with a lower z-value are rendered.
