I don't know if this counts as a joke, but I was there when it happened.
Early morning Physics class filled with slightly dazed freshmen. Eager beaver post-doc teaching the class asks "The wavelength of the Sodium yellow line. What is it? You there!"
Fortunately he has his eagle eye on the guy next to me, who mutters: oh shit, and replies "A hundred and one?"
"Hah!" says the postdoc "A hundred and one what?"
"Um, a hundred and one, point two?"