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.
Other mistake: During the song 'It Takes a Woman' when Michael Crawford is standing beside the white horse, it nips his cheek, before someone obviously pulls it away.