Penn National Gaming sent fliers to 55,000 people, promising $500 worth of free slot machine credits and buffet meals.
The promotion was supposed to have gone to less than 1,000 of the casino's best customers.
Casino officials aren't sure how the mistake happened, but say it occurred at a direct mail contractor.
If all those chits were cashed-in, it could have cost the casino $29 million.
Casino officials say they'll honor $100 in credits and two buffet passes until Christmas.