Setting the Scatter Radius of the Subsurface profile to a really big number will result in rendering errors as you can see in the following image.
In the image on the Left, the Scattering Radius has been set to 50.
Notice how it looks like there are multiple images rendered on the surface.
Now take a look at the image on the Right. In this image the Scatter Radius has been set to 5.0.
Notice how the image looks softer and more natural. This is the type of effect we are trying to achieve.