At one point in my recent class, we surfaced from a dive where we had made a mistake that the instructor KNEW we were going to make, because it's a common mistake to make under the circumstances we were in. (I don't remember the specifics now.) He said, "I could have told you ahead of time to be careful about that, but you'll remember the lesson better because you made the mistake."
And I've been mulling that over since. It's true that making errors -- at least if you recognize them at the time, or shortly thereafter -- tends to throw the experience into high relief and make it memorable. But making mistakes takes a toll on the student, too, in morale and confidence. I've trained animals for years, and I know absolutely that you make better progress if you set them up to succeed, than you do by allowing them to make errors and then correcting them. Although people differ from animals, in that they are taking whatever training they are doing because they WANT to, and they understand the desired outcome, I'm not entirely sure they're all that different from animals in this regard.
I believe there is some educational theory out there that stresses positive reinforcement and structuring experiences to channel the student to success. I'm wondering if anybody here who is teaching technical diving of any kind is using those ideas, or what kind of balance you may strike between permitting the student to make mistakes and learn from them, and creating enough success to keep morale and confidence high.