Wikipedia gives a good summary of the various meanings of that idiom: https://en.wikipedia.org/wiki/Exception_that_proves_the_rule https://en.wikipedia.org/wiki

Wikipedia gives a good summary of the various meanings of that idiom: https://en.wikipedia.org/wiki/Exception_that_proves_the_rule https://en.wikipedia.org/wiki/Exception_that_proves_the_rule Also, I don't think the advice is outdated at all. I've seen it happen multiple times. However, it describes a tendency to be on guard against. It's not some sort of law of the universe. It's also not about rewriting in Go or Rust (you may be thinking of a completely different piece of advice about not doing ground up rewrites.) The second system effect is about being overconfident from the success of an initial system with limited scope, and being too ambitious in a second system.