That is the million dollar question and the difference between a happy customer and a dissatisfied one. Of course there are permanent stains that can never be removed and should be addressed by your carpet cleaning professional once he looks at it. Pet urine stains, any type of artificially colored stains and many others are nearly impossible to remove but can be lightened. Stain kits are available to remove these stains but once again there are factors involved as far as longevity of the stain, volume and carpet color as well.