The President is walking out of the White House and heading towards his limo, when a possible assassin steps forward and aims a gun. A secret service agent, new on the job, shouts "MICKEY MOUSE!" This startles the the would be assassin and he is captured.
Later, the secret service agents supervisor takes him aside and asks, "What in hell made you shout mickey mouse"?
Blushing the agent replied "I got nervous I meant to shout...'Donald Duck'