Yikes. That's a nasty faux pas. Don't think it's lawsuit-worthy, but it does merit some very nasty letters. Not sure about firing the person responsible for the mistake, as mistakes happen, but it is probably something to consider. If they already have a history of sloppy work, this would surely be the last straw. If they're a loyal worker and haven't had a history of mistakes or problems, I might be inclined to reprimand them and let them somehow atone for their mistake (a "letter to the readers" or some such, explaining the error, for example) and put them on probation of sorts.
EDIT: Perhaps having to personally respond to each and every angry letter/phone call/email about it would drive the point home...