 <p>Turns out this was caused by incomplete data entered into the system. Entering a Zip Code was triggering a look-up in the Post Code table. Only this table had blank entries for County. This caused both city and state to be overridden once the user tabbed out of the field.</p> <p>The answer is to disable this functionality. Zip Codes do not respect city boundaries (i.e., there can be multiple cities in a zip code), and they don't have to respect state boundaries either. Having the system override what the user entered in makes no sense.</p>