hi sonita i just stumbled across the answer i think while testing something for someone else.

Check your lights in the parameter dial window(properties part),as one of those lights has got "ray trace shadows" checked i'd guess, now the one with ray trace shadows must of had the "shadow min bias" set to zero on it for that to happen, if you set it to 0.8000etc problem gone.

image is what happens when "shadow min bias" is zeroed.

Hope it helps ya.