jQuery Ajax에서 정상적으로 응답이 왔는데 fail 이 호출되는 경우

Your Ajax request contains the following setting:

dataType: "json"

The documentation states that jQuery:

Evaluates the response as JSON and returns a JavaScript object. (...) The JSON data is parsed in a strict manner; any malformed JSON is rejected and a parse error is thrown.

This means that if the server returns invalid JSON with a 200 OK status then jQuery fires the error function and set the textStatus parameter to "parsererror".

Solution: Make sure that the server returns valid JSON. It is worth noting that an empty response is also considered invalid JSON; you could return {} or null for example which validate as JSON.

You can check whether a JSON is valid or not on jsonlint.com.

출처 : http://stackoverflow.com/questions/6186770/ajax-request-returns-200-ok-but-an-error-event-is-fired-instead-of-success

간단히 번역하자면, dataType을 json으로 해 놓은 경우 반환되는 json의 형태가 올바르지 않을 경우 http response code가 200이 떨어지더라도 fail event가 발생한다. 반환된 json이 정상적인지 확인하려면 jsonlint.com 에 들어가면 가능하다.