When I was a TA in grad school, I had one student who couldn't double click. To double click, she'd take her hand off of the mouse, hover her palm a few inches above the mouse, then stab down twice with her finger. Doing this, she'd move the mouse while clicking the button, so her double clicks never registered. I kept telling her to leave her hand on the mouse and just press twice with her finger, but to my knowledge, she never tried it.
This was the same class where I tried to use the theory of evolution as an example, and about half the class laughed, because to them the theory was just a ridiculous idea. Gotta love the Bible belt!
EDIT: I think there's two keys to the true intuitiveness of an interface that many people overlook. It's easier to make an intuitive interface if you narrow the domain of an interface. It's also easier if you narrow the effective bandwidth of the interface -- show the user more data and they tend to be more confused, show the user less data and they tend to be less confused. Apple seems to apply this in Front Row and in iOS.
> When I was a TA in grad school, I had one student who couldn't double click.
This isn't at all uncommon actually, especially among adults age 50+. Double-clicking a mouse requires a certain amount of manual dexterity, but we've all forgotten that because we developed it a long time ago and to such an extent that it's become effortless.
Recommend they get a trackball; a big, beefy beast that can be slowed to a crawl. Better for the neck, better for double-clicking, and easier for positioning the cursor (especially if they have Parkinson's).
1) Would always miss when he clicked (single click). He'd line it up, eyebrows screwed up in concentration, raise his index finger up high, bit his tongue with intensity, and mash that finger downward. Unfortunately, all the full-body effort of mashing that finger down meant that he skewed the mouse a good few centimeters in a random direction, meaning the entire task started over again.
2) Absolutely did not understand that Wordstar (yeah, you read right) isn't a typewriter. He would laboriously space-space-space-space... manually center text. Hit enter at the end of every line. He would type sentences in -- while staring at the keyboard and using his index fingers -- only to finally glance up and notice an error three lines prior. He would then backspace-backspace-backspace... deleting all the characters until he reached the offending error.
Watching over his shoulder provided perhaps the most fascinating and educational experience I've ever had in user interfaces.