I *think* it has something to do with .wal files. I've had it happen to me, but only with the white and black textures, which are still in the .wal format (ie: keyboard in grave, lights in ub_rooftop, bowling alley in cosmic3). I guess it would make sense if you're loading all .wal files. I haven't seen it happen with jpeg or tga textures, and it doesn't happen often enough to pin down exactly what causes it.