Factual error: In the original theatrical release print of this film, set in the 1890s, there is an overhead shot of a train travelling through the countryside from Yonkers to New York City. As the camera follows the train, an overturned 1950s-60s automobile in a ditch beside the track comes into view. The offending vehicle has been edited out of the video release.
Factual error: Hello Dolly is set in 1890. Barnaby makes several references to seeing "the stuffed whale at Barnum's museum" several times. But by 1890, Barnum's museums had burned down (three times, I think) he'd gone into politics, become a lecture speaker and finally had become partners with James Bailey to create Barnum and Bailey's Circus. (They even have a banner in the parade on 14th Street.) I doubt they still had the museum operating under his name.
Trivia: Three costumes were made for Streisand for the "Hello Dolly" number at the Harmonia Gardens. They are identical except for the length of the train. As she enters she has the dress with the full train. During the number the train "shortens" twice to enable the dance steps, particularly the backsteps, to be performed and for the last few bars she is back to the full train original dress.
Trivia: Director Gene Kelly's first choice to play Minnie Fay was not E J Peaker, but Sandy Duncan, who he had seen dance and sing on Broadway and was perfect for the part - she did a screen test but was turned down. Kelly next wanted Ann-Marget but was also turned down. Peaker went on to a short nine film career, Hello Dolly was her first, she was better known for her TV work.