I noticed the 20% off coupons on the back of receipts this past summer, quite by mistake!! We used them on several occasions and saved a ton of money! On the last day of our trips ( the sad leaving Disneyland Day!) we to go World of Disney to get any last minute things we forgot to pick up during in the parks. Those coupons really came in handy that day!!
I wonder what triggers the coupons to be printed. I didn't look super-carefully on our receipts but I never saw one of those coupons on one of ours. All our meals were for 1 or 2 people.
However, someone was nice enough to leave one neatly folded on the window ledge near the bathrooms in Hollywood Studios at DCA. It was from Award Wieners (I believe it had 3 or 4 meals purchased on it). I kindly availed myself of it.