This is another of those tales I long remember, from high school at the latest. This version was posted on the shaggydog listserv. The author is not known.

Neil Smythe, Professor of Sexual Physiology at Oxford, was lecturing his class. “And so, gentlemen, as you can see through a consideration of the anatomical possibilities, there are precisely seventy-six distinct positions possible in the sex act. If we classify these positions – -”

At this point, however, a deferential French student rose in the back of the classroom and said, “Monsieur le Professeur, I am sorry to be forced to interrupt you, but there are, actually, seventy-seven distinct positions possible.”

Professor Smythe regarded the French student with a frown. “My dear young man,” he said, “my statement reflects long and serious research in the field by many of the most highly respected authorities, men of age and experience. We are ignoring mirror images and trivial variations, of course –”

“Of course, Monsieur le Professeur. But I too, speak with knowledge. The fact is that I, myself — I who am speaking to you at this moment — can personally, of my own experience, vouch for the existence of seventy-seven.”

“Well,” said Professor Smythe, “in a dispute such as this there is an easy way of settling the matter. I will carefully describe the seventy-six distinct positions, and when I am done I will ask you to describe a seventy-seventh, different from all the rest. The remaining students in the class will, I trust, keep careful count and judge between us.”

“Begin, Monsieur,” said the Frenchman. “I will,” said the professor. “We will start with the prime-basic, or common, position: woman horizontal-dorsal, man horizontal-ventral, parallel in line and direction through a vertical axis of symmetry –”

“Sacre bleu,” cried the Frenchman, “seventy-eight!”