I had this once too:
I couldn't render an image at huge size in reasonable time so FrankB helped me out offering to render it. I packed it all up and he returned the render with a different shore as well.
It seemed I accidentially moved the heightfieldshader in the 3D preview, just by 10m, which you can't see by looking at the boundary box.
Therefore I requested in another topic to be able to disable the boundary box of the heightfield, in order to prevent this stuff.
It happens easily because you have to use mouse-buttons to navigate in the 3D preview.
Martin