At worst it would just turn into a coupon or flight ticket that doesn't work, right?
Oh… no… at worst it… would turn into a coupon or flight ticket that DOES work… And then when the true owner of the ticket for B25 on the 4:30 to Seattle comes down the aisle and looks confused to see you in his seat, he'll have something of a time (and the airline a PR nightmare on their hands) if he can't prove he bought the ticket.
Or kids walking out of stores with free game consoles and games thanks to their hacked coupons…
But the coupons can be tied to user accounts and/or be one-of usage. The latter would be case of hacking the algorithm for generating codes. Either way, it's no different than any other coupon scam already in existence.