另當別論 : 另, 別 superfluous? What does 當 add?
另 (adv) 當 (v) 別 (adj) 論 (n)
另當別論 =  treat separately as a different issue = (it is an exception)
Example:
雖然只有工作人員可以進入後台，但有VIP通行證的客人另當別論 - Although only staff can enter the backstage, people with a VIP pass is an exception