The issue is that the line isn't error in all cases. Most of the time only iPixelFormat 1 and the offscreen formats are valid values. But for some queries like how much pixelformats exist a program can pass garbage aswell. This processing is done at a later stage because of this, it could be changed to a warning at some stage.
Roderick
Testing with World of Warcraft, the game runs fine, but I see this in the console:
err:wgl:wglGetPixelFormatAttribivARB Unable to convert iPixelFormat 0 to a GLX one, expect problems!
tom