I used to pour liquid bleach on stains on my Formica.

By "a mystery stain" you mean a stain that THEY did, right? I would insist that they remove the stain on your newly installed counter and if they damage it during the removal, then they replace the counter.

Dilly,
Yes, they did it and they should fix it, however, on each of the three occasions these idiots tried to correct an item on the punchlist, they damaged two additional items. For example, when they put poly on the bedroom floor, they also tracked it across the new floor in the living room and poured it in the stainless steel sink in the kitchen. I refuse to let them into the house again. If I can't clean the corian, I will have it replaced and pay for it out of the money I am holding back from the final payment.