title: When is it appropriate to respond with a HTTP 412 error?
body: <p>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.</p>
  
  <p>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)?</p>