I remember a story about a woman who clipped forty-some coupons from the Yellow Pages that promised $100 off a new car. As the coupon didn't state "only one coupon per purchase" she was certain she was owed a free car for her efforts. She didn't get the car (thankfully), but was exactly was her thought process? It certainly wasn't based on a system of fair play.
I see a difference between ripping off a system due to an error, and making the most out of a system within its rules. The manufacturer and store coupon game is pretty well settled now. Few people are going to actually be able to rip it off due to mistakes, and it's pretty well settled law that the stores don't have to honor mistakes. The best the rest do is play the game to its limits within the rules set by the stores and manufacturers. If the stores and manufacturers don't like it, they'll change the rules.
But you want a real rip-off? Those coupon expiration dates don't mean squat. Soldiers and their families in commissaries all over the world can use those coupons six months after their expiration date! Those unethical people taking advantage of the poor manufacturers.
The dealer blew a perfectly good chance to really get some tremendous advertising, imho. Of course, they'd have to print a notice that no coupons printed before a certain date would be accepted in the future, one coupon per purchase, expiration dates, and all that fine print on subsequent coupons, and run it with their new coupon deal as the story broke in the local news....
A few years back, someone dug an old coupon for a $19.99 Earl Scheib's paint job out of some yellowing publication which had no expiration date on it. They honored the coupon and painted the guy's car for $19,99. The press they got was likely worth multiples of what they lost on that one paint job.