I don't think this is the intent of 412, it's to do request headers that the client specifies, e.g. http://stackoverflow.com/questions/5369480/when-is-it-appropriate-to-response-with-a-http-412-error

Further reading: http://www.bennadel.com/blog/2434-HTTP-Status-Codes-For-Invalid-Data-400-vs-422.htm