Here's a photo from that Consumerist article; a sign posted at a Quizno's saying, or meaning to say, "No coupons allowed":
and yes, this is going to be one of those nitpicky posts where I whine about the lack of educational standards.
"Accept" and "except" are pronounced identically in standard English, which doesn't help matters, but once again we have a situation in which reading makes all the difference. If you've only ever heard the words used, you know what their meanings are, but you might not know that there are two different spellings differentiating those meanings--that they are, in fact, two different words. If you read, you will inevitably have come across the two spellings (they're very common words) and will, even if you are a bad speller, internalize that they are different in a very important way. If you are a bad speller, you might spell the second one "eccept", but at least it will be clear that you understand the difference. If schools taught reading and spelling skills as a matter of course...well, this sort of thing would probably still happen. But not as often.
It's entirely possible, of course, that the sign was written by someone whose first language isn't English, which is a good excuse: English is loaded with sound-alike words that are a trap for anyone who isn't completely immersed in the language. In that case, my whine isn't about the educational system, but about, once again, the necessity for proofreading. If you're making up a sign to be posted in a public place, then you absolutely have to run it past a second set of eyes. Period. I don't care if you're a professor of English: even the best, most careful writer makes mistakes, and it is notoriously hard for the writer to catch them. I also don't care if you're a small business with only a few employees. Make friends with someone who knows the language inside out. Every organization that makes any kind of public announcement--which is to say, essentially, all of them--ought to make it a priority to have on their staff some well-read person who can manage the language.
"Accept" and "except" both have the same Latin root: "capere", "to take". It's the prefix that, as usual, makes all the difference; it acts like a preposition in English and refines the meaning. The prefix to "accept" is "ad-", "towards"; to accept something is to take it towards yourself. "Except" has as its prefix "ex-", which means "out"; to except something is to take it out--out of a list, out of consideration, out of play, out. Therefore, if something is not accepted, it's shunned or forbidden, and if something is not excepted, then it's explicitly permitted, and therefore the sign actually means the exact opposite of what it intends to say.
If I had been in that store, it would have been very hard to resist arguing this point, even though I would have lost the argument.