Mental Ray against Software renderer ???

Hello people

Take a look at these two pictures.One is rendered with Mental Ray and the other is a quick software render.

http://www.slibe.com/image/45b3c0f6-mental_ray_jpg/

http://www.slibe.com/image/0135456d-software_jpg/

Why those strange spots apear when i render with Mental Ray?

You can see the line with software render that is good.Also in viewport that place is ok.No strange thing like in Mental Ray render.

Help apreciated