The Ariane 5 failure is a little more nuanced than that though. The conversion bug was in the Ariane 4 too, they reused the IRS, so it was almost "proven in use

The Ariane 5 failure is a little more nuanced than that though. The conversion bug was in the Ariane 4 too, they reused the IRS, so it was almost "proven in use" but Ariane 5's trajectories put it under much higher acceleration (enough to trigger overflow). It's an embarrassing failure for sure, but it's more nuanced than a "dumb mistake"; the management, testing methodology, and code all failed when it exploded.