It is unclear to me when you should and should not return a HTTP 412: Precondition Failed, error for a web service? I am thinking of using it when validating data. For example, if a client POST's XML data and that data is missing a required data element, then responding with a 412 and a description of the error.

Does that align with the spirit of responding with an HTTP 412, or should something else be used (e.g. another http error code or web application exception)?