NSURLErrorCancelled (-999)

“Returned when an asynchronous load is canceled. A Web Kit framework
delegate will receive this error when it performs a cancel
operation on a loading resource. Note that an NSURLConnection or
NSURLDownload delegate will not receive this error if the download is
canceled.”

你可以做的最好的事情就是忽略这个错误

if ([error code] != NSURLErrorCancelled) {
   //here you'll get a real error 
}