[sqrt17]

A bridge that depends on all rivets being perfect is a bad thing. A proof that no longer holds outside of its domain of application is still useful.

The trick is to recognize when you need one and when you'd rather use the other.

[kiujygtyujik]

Same in software - an application that fills a business purpose but has bugs or "errors" is correct.

An error free perfect piece of software hat doesn't is useless