Harry Potter Effect
The "Harry Potter effect" is a texturing glitch that appears only in newer (4th generation) custom tracks. That's because newer track models are created with the BRRES Editor, that currently has this glitch. This bug won't happen if you use only the SZS Modifier to create your custom tracks.
This gltich happens if the transparent polygons in the BRRES file appear before the normal ones. The previous polygon will always appear on top of the later one, even if it's transparent.
To fix it, you need to have the transparent polygons as the last ones of the BRRES file. To do this you have to set the transparent polygons to be the last ones, in the OBJ file. In 3DS Max you can set the order of the polygons. Google Sketchup uses the texture names for the order, so you need to change the texture names so that they're the last ones.
This will, however, still happen if a transparent polygon is behind another. To fix this, you have to change the order of the polygons even more.