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 student 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”.

Paul Hoffman (1998), The Man who loved only numbers