IT Trainee: OK, so now you press any button to continue

Me (training them): OK

IT Trainee: What do you see now

Me: A black screen

IT Trainee: That's not right, what button did you press

Me: The power button

IT Trainee: Why did you do that?

Me: Because you said press any button.

IT Trainee: Why would you press the power button though?

Me: Because you were vague and assumed they would know what you meant. Expect users to do dumb things.