First time it happened... okay, let's dig in and go deeper.
What are you correcting:
If the same person is found deviating from known rules and procedures an number of times, you need to go a little deeper. It could be that your training methods are not effective or that rules and procedures are too complex. On the other hand, it could be that the person simply misses the competency to be able to operate at the required level or that there is is bigger issue related to his or her attitude. In these cases you need to address how it is that people that do not meet the competence levels are put in this position, or you need to address the issue of professional attitude. When it comes to professional attitude, a key thing to observe here is the person's reaction to being corrected. Being corrected can be painful, and you need to take that into account, but if somebody is open for this correction there is usually not a real problem. If somebody refuses to be open for this correction however, the problem is more serious and it might well be that this is a more serious case.
How are you correcting:
At this point it is important to address the issue completely and deeply. What is the pattern here? Take a person out of the operational proces until you either feel confident that he or she has demonstrated the competence and/or attitude that give the organization the confidence that he or she can function again.
Be mindful of the fact that you are not talking to an evil person here: we all sometimes struggle to learn something, and we all fall into the temptation of ego sometimes (be honest!). But low competence and pretending to be better than you are, or better than your peers or superiors, simply has no place in a high reliability team.
Take a critical look at your training and selection process. Check your supervisor's ability to handle ego-related issues or to judge somebody's competence correctly and fairly. It is not fair or "nice" to put somebody in a job that is above his or her competence!