(A legend has been passed to me in verbal form, and it just occurs to me that I have read it before, except I have completely forgotten about it until this morning. The exact quantities below do not matter.)
A student Alice is finally finishing her very long thesis. Her advisor is, of course, Bob.
On page 50 of her thesis, in the middle of a very technical proof, Alice writes, “Bob, if you get to here, I owe you a case of beer.”
And on page 100, Alice similarly writes, “Bob, if you get to here, I owe you two cases of beer.”
On the day of her defense, Bob walks in to the room and proudly announce to Alice…
“You owe me a case of beer, ha ha ha, gotcha!”
P.S. In this electronic age, care must be taken if there is an electronic copy available. After all, if Bob has the PDF, imagine what terms he may search for… 
