In the "Roof Garden" scene, Nadine is wearing high heels when she does the "Magazine Cover" number. In the wings at the end of it, she removes her headdress, discards her huge fan, and goes back onstage to call Don Hewes out of the crowd to dance their old number. Suddenly, she's wearing ballet slippers. In fact, in all of her scenes with Don, she wears flat shoes, so as not to tower over him.
When Hannah is going to play "It Only Happens When I Dance With You" on the piano, she opens up the music. Music is only on the right side, but in the next shot, there is music on both sides; neither Don or Hannah turned the page.
The feather in Hannah's hat (in the "Fella With the Umbrella" scene) remains relatively intact through the drops to the umbrella. The shot then changes to a closeup, and the feather is drenched. Later, it's standing up straight again, then down again. It's obvious that the closeup shots were filmed before or after the longer shots.
The drumsticks thrown on the floor disappear in the next scene, then reappear in different positions. The drumsticks then reappear in a different position at the next cut.
When Hannah returns to her hotel after Don dances with Nadine and finds Don waiting for her in the corridor, they are on the wrong set. The following scene shows Jonathan arriving at her hotel room the next morning, and the corridor has a totally different decor: different door panel, door-lock, colour-scheme, pictures now on the walls, etc. This set, however, ties in with earlier views of her hotel room.
New York City did not begin using a 2-letter 5-number telephone numbering plan until 1930. In fact, New York City first implemented a 7-character telephone number in 1917, and it consisted of 3 letters and 4 numbers. Thus, the phone numbers Peter Lawford's character asks the operator for, "PLaza-3 1549" and PLaza-3 1598", could not have possibly existed in 1912.
In Mike's bar, a motto is shown with the possessive form of "its" spelled as "it's". While this is incorrect today, spelling the possessive form as "it's" was retained by some to the beginning of the 19th century. Thus, the knickknack easily could have been in the bar in 1912.
A Police officer writes Jonathan (Peter Lawford) a ticket for parking beside a fire hydrant. As he hands over the ticket, he bumps the hydrant which wobbles.
While Fred Astaire's slo-mo dance routine to "Steppin' Out with My Baby" is an impressive technological feat, he is performing it in a live stage show, where it would not be possible to do slow motion.
When the driver gets into the taxi cab, it starts reversing before he has taken the handbrake off or put it in gear.
Most (or all) of the musical arrangements, costumes, and hairstyles date from a much later time period than the setting of the film (1911-1912). This is especially notable as many scenes are stage performances. Although a haphazard attempt is made to make the women's street clothing and hats appear to approximate the 1912 time frame, from the knees down, the light-colored stockings and shoe styles are strictly 1948.
In the opening toy store scene, Don sings about the various toys and points at each. The toy train he points out is a post-war Lionel, but the movie is set in 1911, so it should have been pre-war tinplate or Lionel standard gauge.
During "I Want To Go Back To Michigan", the trombone plays a muted solo, yet the trombone is open - no mute is used.
Marty's hand movements on the piano do not match the notes being played.
While Hannah sings "It Only Happens When I Dance With You", she's supposed to be accompanying herself on the piano, but her hands never reach the low notes that are heard.
When Hannah calls Jonathan on the phone, the shadow of the mic is very visible near a painting.
When Don sings the first line of "It Only Happens When I Dance With You" to Nadine, the shadow of the mic rises from the bottom left-hand corner of the painting above the mantelpiece and goes out of shot.
The stage manager at the Ziegfeld Follies mispronounces "Ziegfeld".
Robert Patrick says in his book, "Film Moi": "The most beautiful goof in movies comes in the 'Steppin' Out with My Baby' number. Fred Astaire at one point goes into slow motion. The chorus behind him continues at normal speed, but that's not the goof. Judy Garland, in the wings watching this miracle, just beams with admiration - as well she should."