Just as we checked the tuning …

In 1853, Brahms went on a tour of German cities with the Hungarian violinist Eduard Reményi.  In the town of Celle, they were scheduled to play Beethoven’s Sonata in c minor (op. 30, no. 2):

but it was found that the piano in the hall was tuned a half tone too low.  Reményi refused to tune his violin down, so, rather than to disappoint the audience, Brahms obliged by transposing his part up a half tone.  He played the entire sonata from memory in the key of C sharp minor.

Leonard, Richard (1961) The Stream of Music.  London: Hutchinson, p. 256.