the exception that proves the rule idiom
反证规律的例外；足以证明普遍性的例外
something that emphasizes the general truth of a statement by disagreeing with it
Most company directors are middle-aged men, but this 28-year-old woman is an exception that proves the rule. 大多数公司的董事都是中年男性，但这位28岁的女董事却是普遍中的例外。