mysterious grainy texture

I have a simple object I made by extruding part of a UV Sphere and when I apply a texture with Ray Mirror enabled one of the faces has a strange grainy texture on it. If you do F12 on the following file you will see it clearly (the render will only take a few seconds).

http://uploader.polorix.net//files/1007/cool8-weird.blend

Any ideas what is causing this?

Thanks!

In Edit mode select all (A) and W >> Remove Doubles.

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Thanks alot Fligh! I never could have figured this one out myself (: