Mark writes in to share this picture.
Here's the thing: if you deal with consumers, you're going to find that some of them are very eager to take advantage of you. And if taking advantage of you means aggressively distributing a coupon, they're going to do it.
Rule #1: don't expect that anything on the Internet won't get out of hand. If you don't want it to get out of hand, it probably will.
Rule #2: if something gets out of hand, and you made a promise, better figure out a way to keep it. This sign is an ineffective response. If it were my call, I'd take advantage of the "one coupon per customer" presumption and put a little tick on the buyer's driver's license or similar... just enough to slow down the particularly egregious scammers (who in this case aren't really scammers. Starbucks asked for it).
Rule #3: have a policy (I know, I hate policies) about internet coupons before someone invents a fake one. You ought to respect the person who traveled to do business with you without doing something that's going to bankrupt you. "Davis Burritos never ever accepts coupons distributed online. If you've got one, it's a fake. But, since you came all this way, feel free to exchange your fake coupon for a free drink with any biggie meal." Put that on a little plaque near the register and you're set for life.
PS When I was in business school, we did the McDonald's case. Part of our preparation was to go to the nearby McDonald's with a stopwatch and clipboard. We walked in the door and stood just long enough to get noticed. Boy did those guys hop to attention. Then we went to another McDonald's and performed the following experiment (please, please do not try this at home, just take my word for it). We ordered a milkshake and a Big Mac. Ate half the Big Mac. Drank half the milkshake. We put the Big Mac remainder into the milkshake cup and went to the counter, "I'm sorry, I can't drink this shake, there's a Big Mac in it." They gave us a new one.
Because McDonald's didn't want counter people making decisions about who to say "no" to. It was worth the expense of humoring idiots like my study group for the brand power of knowing that counter people didn't alienate people on a sliding scale.
I think they should have called the cops on us, but you get the idea.