I used to work in a hospital eye clinic. As I was setting up the equipment for a visual field test, the patient asked me how long I'd been doing this. I jokingly said - "Actually just since lunch time. Normally I work in the booth in the parkade, but they're very short staffed here. "
The guy didn't miss a beat. "That's okay, I'm doing this test for my brother."I laughe for days.

Another one:
My love comes home from the video store. "I got Quigley Down Under," says she.
"You should get some penicillin," says I.