There's a pretty good chance the person who made such a mistake is less likely to repeat it or other mistakes than someone who hasn't yet had the experience. So at best you're just sending a more valuable employee to one of your competitors.
There's some gray area when it comes to making a poor judgement call to skip a well defined procedure or be lazy.. and some cases this doesn't apply to like intentional malice or a continued series of similar obviously avoidable mistakes.
On a related note im often frustrated when people play a blame game (even if one might be somewhat called for, in a way) instead of considering "human factors" and how to systematically avoid people being able to make such mistakes. Or refuse to accept a human factors style explanation and just say the person "should have known better" etc. The best high profile example was that hawaii missile alert that was supposed to be a test having an incredibly poor UI. But there are countless examples everywhere all the time. I often try to think about this a lot in my work.