They'll also be the person most able to identify what went wrong with your processes to allow the failure to occur and think through a mechanism to systematical

They'll also be the person most able to identify what went wrong with your processes to allow the failure to occur and think through a mechanism to systematically avoid it happening again. Also, they're probably the person least likely to make that class of mistake again. If you can keep them, you've added a lot of experiential value to your team.