Hey...
a group of mine is playing with mods and for some reason i am the only one with a problem where on certain parts are no textures... and not even google gave me any hope since i seem to be the only one having that issue. [Playing on 1.6.4]

Pic:
http://oi59.tinypic.com/14ctgtk.jpg

I do recall this bug, but I don't remember if it is something I ever solved. Are you using Optifine or MCPatcher?

Well i fixed that issue with simply deleting my whole .minecraft now but just to answer your question Optifine.