After Tootie finishes singing "I was drunk last night, dear mother" you can see and hear Esther (Judy Garland) begin saying a line: "Tootie, you're a very bad--". The shot immediately changes to a closeup, and she repeats the line, this time more audibly and in its entirety: "Tootie, you're a very bad girl!"
As Esther comes down the stairs to the party in the parlor
(with John Truett as one of the guests), she passes by the grandfather clock on the landing. In the shot just before coming to the landing, the pendulum is swinging. In the next shot, Esther is on the landing, and the pendulum is stopped.
In the supper scene when they are trying to rush the meal so that Rose can take her long distance call in private, Katie the maid serves Mr. Smith his soup. The bowl looks completely empty through the whole scene until Katie comes to pick up the bowl. At that point, it is full of brightly-colored soup.
After Lon's going away party when Esther gave John back his hat, he found raisins in his hat. He then walked outside, but after Esther called him back it appears that John is no longer holding the raisins.
When Esther and her friends are boarding the trolley, someone says "The exposition doesn't open for six months." According to the title card, it is "Summer 1903". In the final scene, when the Smith family is going to the fair, the title card reads "Spring 1904", closer to 9 or 10 months later than summer 1903.
When one of the girls in Esther's group of friends going to the Fairgrounds in the first act of the film says the site must look like a fairy land, one of the boys tells her the fair won't open for six months. This scene took place in the "Summer 1903" act of the film and before the "Autumn 1903" act. The Louisiana Purchase Exposition opened in St. Louis in April of 1904. If the fair wouldn't open for six months, the excursion of Esther's friends to the fairgrounds should place the date of that scene sometime in November of 1903.
Early in the movie the father mentions the Baltimore Orioles. By mid-1903 (the approximate time at the beginning of this movie) while there was no major league team named the Baltimore Orioles there was a AAA team the Baltimore Orioles. In fact when the movie was filmed, the Baltimore Orioles still did not exist as a major league team (would join the American League in 1954, coincidentally moving from St. Louis where they were the St. Louis Browns).
It is often incorrectly claimed that an off screen male voice calls out "Hiya, Judy" (referring to actress Judy Garland instead of her character, Esther). The voice actually says "Hiya, Johnny". This refers to Tom Drake' s character, John Truett, who has been trying to catch the trolley and apparently just made it. As soon as the line is delivered Esther looks expectantly screen right but we do not see John until the end of the trolley song sequence.
The goof item below may give away important plot points.
When Esther and John are looking out over the fair at the end, John remarks that he liked it better when it was a swamp and it was just the two of them. They didn't actually meet until the night before the trolley trip to see the fairgrounds, already in progress.