I load a texture into OpenGL. I map it on a bunch of triangles. Stuff works. Now, I turn on fog. All the triangles that are only partly on screen have their textures all messed up... Any ideas? Thanks

I had the same problem a few months ago.
Check out old thread here on gamedev: Found OpenGL Bug!!

My guess is you have color depth set to 24bit. OGL probably doesn''t support 24bit color depth.
If you use 16bit or 32bit all is fine again (and faster)