Engine failure with loss of thrust on final approach/in landing configuration
Never ceased to amaze me how many folk in the box took forever to realise that one had run down ... and often too late to recover the situation ... a couple of practice looksees and it doesn't catch them again ...
I'm with Centaurus on silly systems failures .. but there is some value in prebriefed, no jeopardy, unusual handling problems .. even if only for the value in a pilot's having had a look at the situation. If, on the other hand, there is any associated pressure by the back-seater, most of the value evaporates ... as the guys end up worrying about what the instructor is thinking rather than using the exercise as a self-exposure/extension sort of thing.