Add to Cart Error Message

[cyberniche]

I am receiving this message when I (or customers) click on the "add to cart" button.

ShopSite detected that the information passed from your browser is incomplete and cannot be processed. The information passed is:
Reg.htm

It doesn't happen on all products, just one of them. How would I go about fixing this?
Thanks.

[Jim]

Are you using the Order Anywhere feature and putting the add to cart buttons on a page not created by ShopSite?

Does the page that the product is on have a custom made form for search or customer registration?

A link to the page where the problem is occurring would be helpful.