I did a form that worked wonderfully... you entered a zip/postcode (first, before country and such). The form would look it up in a background ajax call. It wo

I did a form that worked wonderfully... you entered a zip/postcode (first, before country and such). The form would look it up in a background ajax call. It wouldn't tell you this or block your actions. If it found a match in our DB, it would populate the city, country fields, not make a mess, and leave you free to edit the form if it was wrong. Your flow would be the same either way. I think not interrupting the users flow was a big part of it.. it didn't make people go "What? How do I change that? What happened? That's not right".. you just kept filling it out like normal... with some fields pre-filled if you were lucky.