When attempting to parse JSON returned in a non 200 fashion, i.e. a 400 error, calling the JSON.parse(data) does not work on the failure, while it works on the success version. The work around for the failure is to pass the responseText.

$.post("/someservice", json).done(function(data) {

var response = JSON.parse(data);

DO something

}).fail(function(data){

var response = JSON.parse(data.responseText);

Do something

});