Prior to version 6, iOS didn't honour the timeout flag set on a url request. For this, the library added code to care for this situation (by attaching a timer on the request and on a timeout manually cancelling the request and emitting the failure block with error code '-1001' [request time out])

On its current version, AFNetworking when receiving a cancel message it too emits the failure block but with different error code -999 [request couldn't be completed]

This has the effect:
a) duplicate call of the failure block
b) different error codes. On iOS 5 we get '-999' and on iOS >= 6 '-1001' [request time out]