422 error unprocessable entity code example

Example: 422 (unprocessable entity)

To reflect the status as of 2015:

Behaviorally both 400 and 422 response codes will be treated the same by clients and intermediaries, so it actually doesn't make a concrete difference which you use.

However I would expect to see 400 currently used more widely, and furthermore the clarifications that the HTTPbis spec provides make it the more appropriate of the two status codes:

The HTTPbis spec clarifies the intent of 400 to not be solely for syntax errors. The broader phrase "indicates that the server cannot or will not process the request due to something which is perceived to be a client error" is now used.
422 is specifically a WebDAV extension, and is not referenced in RFC 2616 or in the newer HTTPbis specification.
For context, HTTPbis is a revision of the HTTP/1.1 spec that attempts to clarify areas that were unclear or inconsistent. Once it has reached approved status it will supersede RFC2616.