The Vincente Minnelli musical is often considered one of the best of the genre and, as with every film—accomplished or not—it has its share of mistakes. Nanette Fabray, on one of The Band Wagon's DVD extras, recalls a doozy that occurs during the "Louisiana Hayride" number: "You must be very, very quick and you will see a stagehand walk through the back of the set. If you're watching the picture, see if you can catch that stagehand. I would guess he assumed that they had moved away from where he was and he just took off. Nobody caught it."
The stagehand appears at the 4:27 mark.