If any interference with the controls or anything else happened because the student didn't meet the standards (i.e. acted downright unsafe or potentially unsafe), that should have never been kept quiet about by the instructor. That's what an instructor is for - ensuring that the process of training is not just some formality but a path towards achieving consistently safe performance well within any existing margins and tolerances. If the student is underachieving somewhere, that's never best dealt with by quietly fighting him on the controls and not giving any feedback and guidelines for rectification of the problem.
This.
Maybe he felt you needed 10 degr of flap because of the runway remaining or lack thereof or obstacles in the climb out path.
Maybe he felt like being helpful in turning your landing or taxi lights on as you were entering the runway and you forgot.
Maybe he turned off the nav lights that you mistakenly turned on.
Though I generally agree with B2N2, I don't agree on these points. None of those three examples are make or break for a safe flight. Yes, perhaps the alternate selection would have been preferable, and a discussion is warranted. But in the mean time, the candidate pilot is either adequately safe or not. If they are adequately safe, and some counseling will make them safer, excellent, counsel in a useful way, without needless task saturation or cockpit confusion.
If an instructor has let things quietly get to the point where an error is now a safety issue, the instructor was not proactive. An example is: When I train amphibian pilots, I will prebrief very specific memory action items to do with landing gear position selection and verbal acknowledgement of the check. I will inform the candidate that I will give them
one courtesy verbal reminder - I will not touch the landing gear selector at all. My verbal reminder will not be to select the landing gear, but rather than they failed to observe and state the position, and landing surface. I still will not touch the landing gear selector. When they forget this a second time, as briefed, I still will not select the gear for them, that's teaching them to forget to do and check for themselves, I will call an overshoot at a suitably late stage in the final approach, as I have briefed that I would do. I will select a point on final where an overshoot is safe, and in the case of an engine failure, a safe landing could still be made. but the point is that it's memorable for them - that's how pilot's learn. Muscle memory is built up because the
candidate's muscles did it.
If an instructor starts doing things in the cockpit, particularly un noticed, there is no learning for the candidate, but rather confusion. Either they learn that they can forget, 'cause someone else will notice and correct things, or, they will be slow to absorb the seriousness, and develop their own rhythm of do and check.
Sure, we all forget things from time to time. Task saturation does not help. The key is to build a foundation in a candidate so they are less task saturated, to the point that they have the entire plane handled. If a candidate is too task saturated, or otherwise inadequate, then correct, don't sign them off for the rental. If they are adequate with exercise repeat or post flight briefing, then do that. If you, the instructor would like to help, brief that in advance, as an offer to reduce the candidate's task saturation, so they can focus on the primary task. Example: "This looks like a really gusty crosswind landing, Would you like me to select the flaps and xxx as you call them out?".
Hopefully, the instructor is ahead of his/her game well enough to anticipate, and allow for variability of candidate pilot behaviour, to maintain safety, yet prevent even more task saturation and cockpit confusion.
I've had a few pilots who I would not sign out. That always came after multiple briefings, and my offer of assistance in the cockpit to help them focus. They just could not manage the handling or complexity of the airplane. For the few times in my career I've had to take something over (always flying the plane on the whole) I always felt badly, as I realized that I had left things too late to be able to brief the next action so the candidate could learn from what was about to happen. I took over and they stopped learning... they are there, paying to safely learn....