gift card problem
 

In conjunction with a slot tournament in Las Vegas, I got 6 $25 Harrahs Gift Cards (I think they transact under Mastercard). 5 of the cards worked correctly, when I tried to use one of them at Ballys casino, it showed that it was used earlier that day (also at Ballys). The cards were in my pocket(grouped with a rubberband) at all times, NO ONE (except for me) had access to them. $25 was loaded into each of these on 6/16 and the problem occurred on 6/20. How could this have happened?
Some possibilities(?): (1) two cards with the same number and CSV number on the reverse were issued, (2) the card info was manually entered (as opposed to being swiped at a terminal), and some digits were miskeyed.
I contacted customer service and they are investigating.