Some excerpts that might eventually come in handy

Eins. «For all of Kummer's work on the cutting edge of number theory, he was apparently rather bad at elementary arithmetic. One story has him standing before a blackboard trying to compute 7 times 9. "Ah," Kummer said to his high school class, "7 times 9 is eh, uh, is uh..." "61," one of his students volunteered. "Good," said Kummer, and wrote 61 on the board. "No," said another student, "it's 69." "Come, come, gentlemen," said Kummer, "it can't be both. It must be one or the other." (Erdös liked to tell another version of how Kummer computed 7 times 9: "Kummer said to himself, 'Hmmm, the product can't be 61 because 61 is a prime, it can't be 65 because that's a multiple of 5, 67 is a prime, 69 is too big—that leaves only 63.'")»

Paul Hoffman, The man who loved only numbers: the story of Paul Erdös and the search for mathematical truth. Hyperion, NY, 1998, pp. 208-209.

Zwei. "… The propagation of unchecked and uncheckable anecdotes about the history of mathematics is a form of pollution to be combatted. An occasional tall tale, with appropriate caveats, can certainly be used to spice up the exposition from time to time, but when no sources are given for anything, such tales become an unacceptable norm."

H. M. Edwards, Review of three popular books on the Riemann Hypothesis. Math. Intelligencer 26 1 (2004), p. 57.