Factual error: In the 19th Street Parade, it shows the Budweiser Clydesdales pulling their famous wagon. Even though this movie is set in 1897, Budweiser did not start using Clydesdales until April 1933 when the sons gave their father, August Busch Sr., the owner of Budweiser, a 6 team team of Clydesdales to celebrate the end of Prohibition.
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.
Continuity mistake: During "Put on Your Sunday Clothes" as the men dance downhill the camera sweeps across the lake and a roadway with cars is visible. (00:33:07)