Chaplin & John Philip Sousa, 1916

Sousa described the event in his autobiography Marching Along:  

It was at the Hippodrome Sunday feature concerts in 1915 [sic] that I first met that public idol, Charles Chaplin.  We had been reveling in the vocal gifts of Melba, Culp, Garden and Fremstad.  Charlie was therefore quite a departure.
“I want to lead your band!”  said Charlie.
“In what number?”  I asked.
“The Poet and Peasant overture,” he confidently replied.
At the rehearsal he mounted the podium, took my baton and as the band started the stately measures of the opening, he proceeded to beat time fully four times too fast!  That well-known blank expression came over his face but this time it was involuntary.  “That isn’t it!” he exclaimed.  I smiled.  “But I’ve played it many years,” I reminded him.  Suddenly I realized that he remembered only the allegro and had forgotten all about the moderato, so I told the band to begin again, this time with the allegro, and we were off!  On the night of the performance, the audience, reading his name on the program and never having seen him in the flesh, suspected a trick—-some clever impersonator of Chaplin—but, as he came from the wings, he did his inimitably funny little step and slowly proceeded to the band platform.  The house, convinced, rang with applause.