Thanks for the response again.

Changing the settings to high and production worked. I've changed them back now, and it is still working - hopefully it won't appear again.

Another question - what is the affect of the bias option that you talk about?

And one more - the tutorial I'm doing tells me to click on the LightFog paramater under light effects for my selected spot light, and the text "lightFogSE1" appears in the box. But, there is not change to my render - the fog that appears in the tutorial's image does not appear for me.

Any ideas on this?

Thanks again,

g.