1. Common Errors in English: About exception proves the rule
The Latin original of this saying dates back over two millennia to Cicero. It means if you make an exception to a rule, a rule must exist. If you say “in case of fire students may use the emergency exits” it is clear that the rule is that normally students are not supposed to use those exits. Few people understand this point and they misuse the phrase “the exception proves the rule” to mean that a rule is not really a rule unless there is an exception to it. This makes no sense. It’s better to simply avoid this misleading phrase.
ㅡ Claim: The exception proves the rule means "counter-examples verify the adage's claim"
ㅡ That's TURE, but not for the reason you think
An instance that does not obey a rule shows that the rule exists. For example, "John's much shorter than average but excels at basketball—the exception proves the rule." This seemingly paradoxical phrase is the converse of the older idea that every rule has an exception. [Mid-1600s]
Prov. Something that does not follow a rule shows that the rule exists. (Often used facetiously, to justify some rule you have proposed but which someone else has listed exceptions. From a Latin phrase meaning that an exception tests a rule.)
ㅡEllen: Men are always rude.
ㅡJane: But Alan's always polite. And Larry and Ted are polite, too.
ㅡEllen: They're just the exceptions that prove the rule.
ㅡBill: All the shows on TV are aimed at people with low intelligence.
ㅡAlan: What about that news program you like to watch?
ㅡBill: The exception proves the rule.
This is often used to mean that the exception confirms the rule. However, prove in this case means 'test'; the fact that there is an exception implies that the rule may not be valid, or may need some modification. The phrase is perhaps most often used as a more or less meaningless reply to an inconvenient fact: 'You're always late'. 'I was early this morning'. 'Ah, but the exception proves the rule!'.