I think of myself as a problem solver rather than a disciplinarian. When mistakes happen, I always ask the people involved for their take on what happened, and why. Often in telling me this, they will also propose a solution for avoiding the same situation in the future. I also find that lots of thespians are like you, GalFriday; they’ll beat themselves up about a mistake more than I ever could. I use this approach with actors, crew, and SM team. If the people involved don’t come up with their own solution, I’ll talk them through what steps to take in the future. I find actor personality conflicts, people who have a bad attitude, or those who are just unsuited to the work of stage management to be more frustrating and less “solvable” than concrete mistakes like forgetting to check a preset or dropping lines.
I’m usually not in a position to take putative measures, so if the same mistakes continue to be a problem, I’d take the issue to the production manager. With crew or SM issues, I can sometimes re-assign jobs if I really don’t trust someone to improve. With actors, I can’t perform for them, so the best I can do is ask for their cooperation. I mostly work on plays with a fixed end date, so in the worst situations at least I know I only have to deal with the “problem child” for a limited amount of time.