The ur-example is probably Ancient Rome. Their Republic lasted for centuries, and the Roman Senate famously had zero formal power. When the Senators were doin

The ur-example is probably Ancient Rome. Their Republic lasted for centuries, and the Roman Senate famously had zero formal power. When the Senators were doing their jobs well, it was held in extremely high regard, and Rome did (via some thin "who was elected to do that this year?" details) as its Senate advised. When not - well, things eventually went downhill. And "Republic" was replaced by dictators-for-life, who got the title "Caesar". During the downhill slide, the Romans tried to codify loads of their previously unwritten rules - about how gov't officials were chosen, what they were allowed to do, etc. That really didn't help.