Three guys(a regular Joe Blow,a musician, and a diabetic) walk into a bar.
The J.B. orders a beer.
The musician orders a scotch on the rocks.
The diabetic orders a diet coke.
Half a dozen beers later,the J.B. climbs up on the the bar and begins belting out a drunken melody. Soon, people begin to leave the bar. More people leave, until only the musician, the diabetic, and the bartender remain. The J.B. changes his pitch, and the musician claps his hands over his ears and rushes out. Only the diabetic remains, and he calmly finishes his diet coke, seemingly unfazed by the horrible caterwauling. Eventually, the J.B. gets tired of singing and shuffles out of the bar. The bartender asks the diabetic why he didn't leave.
"Wrong type of key tone" replies the diabetic.
(yeah, pretty lame but its my first attempt at creating a diabetes joke..)