It's easy to say "shouldn't happen" and "should give students warnings and instruction" but in the real world all it takes is a distraction or some unusual small event to interupt a pilots train of thought and - zip - done in an instant.
A friend of mine managed to do it in his own machine when he was 500+ hours experienced. Simple distraction at the time - can't recall the full details but enough of an overspeed to have to have the engine pulled out.
Don't know what the answer is, but I know it isn't just instruction or checking out the pilot beforehand. Instruction etc stops them doing it 99.99% of the time but you need a 100% system.
The sticker probably isn't the answer cos there are a number of times when it can happen so you'd need a sticker for every situation (and with all the stickers we have to have I don't think I've got room for any more
)