My explanation? Hacks.
10th January 2008
Yah, pretty much my textures show up fine in radiant, but not in game. I've placed the textures in the .pk3 file of my map, any help>
2nd October 2005
Make sure the texture is a power of two (size wise)... this is usually the most common problem.
8th October 2005
For full details, check out the link in my sig about custom textures.