Regarding carpet: There seems to be no cure for this type of stain. However, on hard surface I heard if you place a bleached and water, soaked towel over the spot and leave it overnight, this helps remove the yellowing, sometimes completely. Also, word of advise; the stain is a result of the rubber backing breaking down from contact with trapped moisture between rug and surface. The moisture causes the glues to breakdown in the rubber and tranfer onto the surface, thus leaving the yellow stain. I always suggest using only 100% latex backing area rugs. There not suppose to break down and cause this problem. Hope this helps!