Understand it's not literal; it points to unusual cases highlighting a rule's general truth. In a sea of tech enthusiasts, her indifference to smartphones is the exception that proves the rule.
Pro Tip 2/3
Not Justifying Errors
It's not used to excuse mistakes but to emphasize the reliability of a general pattern. He rarely makes spelling errors, his one mistake was the exception that proves the rule.
Pro Tip 3/3
Irony and Surprise
It can express irony or surprise when an unlikely event occurs amidst the usual pattern. A sunny day in the rainy season could be said to be the exception that proves the rule.