That's a far more dangerous territory. A machine that is obviously broken will not get used. A machine that is subtly broken will propagate errors because it will have achieved a high enough trust level that it will actually get used.

Think 'Therac-25', it worked in 99.5% of the time. In fact it worked so well that reports of malfunctions were routinely discarded.

There was a low-level Google internal service that worked so well that other teams took a hard dependency on it (against advice). So the internal team added a cron job to drop it every once in a while to get people to trust it less :-)