Here's a couple I've come across on the web:
I was performing a complete physical, including the visual acuity test. I placed the patient twenty feet from the chart and began,
"Cover your right eye with your hand."
He read the 20/20 line perfectly.
"Now your left."
Again, a flawless read.
"Now both," I requested.
There was silence. He couldn't even read the large E on the top line. I turned and discovered that he had done exactly what I had asked; he was standing there with both his eyes covered. I was laughing too hard to finish the exam.