On one call I specifically made a point of explaining that the screen and the computer were separate, and that the computer would be under the desk. After a very frustrating 15 mins of trying to get the user to turn the computer on, in desperation I asked them how far apart the button they were pressing was from the screen.
"Oh, it's on the screen"
*mute phone, bang head on desk
The call was resolved quite quickly after I explained a second time the difference between a screen and a computer...