There are several places already reporting this news, & this just doesn't look good for HP. They've even resorted to calling customers & leaving voice mail asking for the extra money or the return of the products.
Would you like to know more? I could understand if they made this mistake for a day or so, but to expect people to pay up or return after this long? The best thing HP could have done was to fix the pricing error & honor what they'd sold, or at least shipped already (& let you know it was sold at a much discounted price). At least they've earn some goodwill that way.