How to fix “NSURLErrorDomain error code -999” in iOS

The error has been documented on the Mac Developer Library(iOS docs)

The concerned segment from the documentation will be:

URL Loading System Error Codes

These values are returned as the error code property of an NSError
object with the domain “NSURLErrorDomain”.

enum
{
   NSURLErrorUnknown = -1,
   NSURLErrorCancelled = -999,
   NSURLErrorBadURL = -1000,
   NSURLErrorTimedOut = -1001,

As you can see; -999 is caused by ErrorCancelled. This means: another request is made before the previous request is completed.