I always thought it meant "the exception (to the rule-of-thumb) proves the (hard, correct) rule".
It's used in multiple senses, so it's almost impossible to tell what the person using it means.
https://en.wikipedia.org/wiki/Exception_that_proves_the_rule...
alt Hacker News
It's used in multiple senses, so it's almost impossible to tell what the person using it means.
https://en.wikipedia.org/wiki/Exception_that_proves_the_rule...