What causes the Landscape checker effect and how do I get rid of it.

Hi Guys,

I’ve had a problem come up today (amazingly for the first time) but can’t seem to find any way to fix it.
I have a landscape in my scene and for some reason just one of the component squares is rendering completely differently to the rest.

As you will see in the image below, the square at the lower right of the image has completely different lighting to the rest of the shot.
After some experimenting I’ve discovered that…
You can see the problem in Lit and detail lighting display. modes.
You can not see the problem in unlit or lighting only display modes.

If anybody knows why this is happening and how to fix it I would be eternally grateful.