be the exception that proves the rule

To contradict a rule and thus confirm that the rule exists. A: "We're always told to get eight hours of sleep, but I usually feel really groggy when I sleep that much." B: "Well, I guess you're the exception that proves the rule." A: "Video games are all just mindless filth that rots kids' brains." B: "I don't know, a lot of them let kids express themselves creatively or learn about the world in new ways." A: "Bah, those are just the exceptions that prove the rule." A: "But I hardly ever study for tests, and I get good grades." B: "Well, you're the exception that proves the rule—it'll catch up to you eventually!"