I would talk to them about it at check in...wouldn't waste a long-distance call on it, but I would tell them about it. Who knows, maybe it's not an error, and you get a bonus. Or, they already are correcting it...I would not count on the error sticking though. Accounting has a way of catching these things. If you point it out & they let it stay, bravo. If not, pat yourself on the back for being a good person.
I'm basically an honest person, and would hate for someone somewhere to "get in trouble" for messing up something...firm believer in karma...what goes around comes around, and do unto others, etc.