'Fatty and Mabel at the San Diego Exposition' is a 1915 silent short comedy film, directed by Fatty Arbuckle and starring Arbuckle and Mabel Normand. There is a brief scene of a Chaplin impersonator (Harry McCoy) in the crowd.
Arbuckle and Normand followed the Keystone tradition of showing up at an actual event and using that as background for a largely improvised film. The event in this case was the Panama-California Exposition, held in Balboa Park in San Diego, California in 1915-1916.
You can view this fascinating footage on Youtube right here: