Senario:

You are the chief aeroplane washer at the company hanger. You have to:
(1) Hook high pressure hose up to suds machine.
(2) Turn on.
(3) Get an important call and have to leave.
(4) As you go home early, yell to Don, your assistant, "Don,...turn it off."
(5) Assistant Don, thinks he hears, "Don't turn it off." shrugs, and leaves the area right after you.

Result:

Well, at least they're clean...!

Do You need to be instrument rated to taxi out of the hanger?