I have about 2000 hours of instructing including PPL, CPL, IRT, night rating, Tailwheel conversion and aerobatics. About 60 of the 2000 hours was initial training for the multi rating. ALL of the really scarey moments instructing were in that 60 hours. They included
1. A student inexplicably aplying full up elevator when I failed an engine during a simulated overshoot (at altitude thank god ). The airplane instantly stalled and snap rolled. I took control with the airplane inverted and pitched 45 deg nose down
2. A X airforce lightjet pilot who on short final got the knobs mixed up and pulled the blue ones all the way back when he meant to reduce power
3. A student who selected gear up instead of flaps up during a touch and go ,thereby writing off the airplane and ending 13 years of accident free flying for me
( Thats why I never do touch and go's in complex airplanes anymore )
My advice.
1. Fully brief your flight and be very clear about what you exect the student to do.
2. Don't fail any engines close to the ground
3. Don't let your guard down for a second. It is incredable how fast things can turn to S***