From time to time we may add new fields to our Flex APIs. Should your API call include the extended option "includeNewFields", then any such fields will be returned. If your code isn't handling a new field correctly, we recommend you either change the code for the new field, or change the request to stop sending the "includeNewFields" parameter. We also recommend you review your code and/or XML parsing library and make it lenient for unexpected fields.
Please also refer to Flex APIs Version Policy (flightstats.com).