E. Kummer, the German algebraist, was so bad at arithmetic that he used to ask his students to help him with the simplest calculations.
Once he tried to find 7 × 9. He hesitated, and then wrote 61 after a student suggested it. Another student corrected him, saying it should be 69. “Come, come gentlemen,” said Kummer, “it can’t be both – it must be one or the other.”
Kummer had a predecessor in this respect. According to Spence’s Anecdotes, Sir Isaac Newton was so deep in algebra and fluxions that he could not readily make up a common account, and when he was Master of the Mint he had to get someone to help him with his accounts.
Source: The Penguin Book of Curious and Interesting Mathematics