In my application at first time reverseGeocoder results like error block below :
didFailWithError:Error Domain=NSURLErrorDomain Code=-1011 \"The op
Error -1011 is Bad Server Response (from the apple docs here) and the HTTP status code is 503 (Service Unavailable).
So, I guess that unless you're giving them invalid data, it's a problem their end!
However, there are a few edge cases that you might consider checking :
(1) What's the value of coordinate? If it's invalid then the geocoder probably returns some sort of error (though it probably shouldn't return 503 ;)
(2) Are you behind a proxy / web authentication - if you are then you're not actually talking to a geocoding service, you're talking to a proxy which probably won't understand what you're trying to do!