I had the same thing happen to me in the Ocean Bar once. But in my case, it was only a matter of an extra punch or so. I didn't say anything, because I honestly could not be 100% sure that the number of punches on the card was wrong, but since then I've always put my initials in a corner of the card.
A related tip ... not so much for cruising, but important in daily life ... when you use your credit card at a restaurant, always check it when it is returned to you just to make sure the server didn't get his hands crossed and give you another person's card. Some of the cards are identical and you honestly might not notice for several weeks that your charging stuff on someone else's card ... and presumably he on yours.
Blue skies ...