People will always make mistakes and some will always be gullible to try anothers weird suggestions it's human nature and if you really think that just because there is a sign saying Instructors/Examiners then none else shall read it then you are sadly mistaken.
TRE's IRE's et al. run out of fuel or forget the gear as do mere mortals and students and others try inverted spins when erect was all the P.O.H. cleared it for and solo students do try to formate and climb into airways and push on into cloud and some get away with it and never think the better of it . Some scare them selves silly and learn a valuable lesson that they should never have had to the hard way with the worst outcome being a broken aircraft and a damaged pride and others sadly die. If you feel they should never have gotten into the air in the first place and that instructors should be able to 'see' those traits and stop them before they ever got to that gullible stage then you are mistaken in my opinion. It is up to the those who can to pass on the wisdom, and not to just throw things out there that they know to be wrong, indeed dangerous, without a measured and thoughtful explanation. Unless of course you are asking the question in the first place in which case lets hope you get a measured and thoughful response.
If you do not think students take things at face value then I wonder how many students you've taught especially without the privelage of having English as their first or even second language. If you are not clear and concise and deliberate in your choice of words with the brief and post flight discussion then you'll be in for some very interesting times and once you wonder how you got there and ponder your own actions and words you'll eventually realise the student did exactly as instructed.