The film is set in the immediate aftermath of the release of The Jazz Singer with its industry-revolutionizing use of synchronized sound. However, dialogue dubbing of the sort central to the plot here was not possible until many years later.
When Lina meets Don at the R.F. Simpson's party after the big premiere of "The Royal Rascal", she mentions something like "I didn't see you last night at Wally Reid's party". Wallace Reid died in 1923 and the action of the film is set in 1927.
The theatre where Vitaphone is playing has a wedge shaped marquee, with milk glass readerboards and blank letters. Trapezoidal Moderne marquees didn't show up until 1935/36, wedge shaped marquees showed up later than trapezoids, and illuminated readerboards with silhouette letters even later than that, so although typical of the early1950s era in which the movie was filmed, it's otherwise at least a decade ahead of the late 1920s era in which the story is taking place.
When Lina Lamont is having problems talking into the microphone hidden in the bush and Roscoe Dexter and the sound man are in the booth, the sound man shakes his head and says, "She's gotta talk into the mic, I can't pick it up." His mouth does not move as he says this.
R.F. Simpson tells his guests that Warner Brothers are making "a whole movie" using the new talkie system. He is referring to The Jazz Singer, which is mainly a silent movie. Only a small proportion of it contains sound.
After the premiere of "The Dancing Cavalier", Kathy (through Lina) announces to the audience that they will hear a live performance of "Singin' in the Rain" in the key of "A flat", but the song is actually performed in the key of "E flat".
When Don Lockwood is trying to create the romantic scene in the empty stage set, he says "We add five hundred thousand kilowatts of starlight" and flicks the switch to turn on the lights. "Five hundred thousand kilowatts" is a gross exaggeration as, even if there had been five hundred lights (highly unlikely), each would have had to produce one megawatt of energy, which would have immediately melted all the bulb filaments (and would have been impossible using 1920s technology anyway).
After the "Beautiful Girl" number, Mr. Simpson discusses hiring Kathy with Sid Phillips and Zelda Zanders. Sid Phillips delivers the line "Mr. Simpson might cast you as Zelda's sister." This is followed by an unusually prominent clicking mouth twitch, presumably out of character.
During the Cyd Charisse nightclub dance number when she's wrapped around Gene Kelly, her body completely changes position between frames due to a clumsy edit. According to commentary on the special edition DVD, this cut of only a few frames duration dates back to the original release of the film and no one knows why it exists.
In the "Moses Supposes" scene, the book that Cosmo Brown throws over his shoulder can be seen on the floor behind and to the left of the desk. By the end of the song, the book is gone and a previously not-present small trash can is now on the left side of the desk.
The scene with Gene Kelly performing an action sequence from the silent film "The Royal Rascal" uses footage from his earlier film The Three Musketeers. After he throws the guard with the spear over the stairway railing, Lana Turner, who played Lady de Winter in the earlier film, is briefly seen coming through the door on the landing before it cuts to new footage of Jean Hagen hugging Kelly. The discontinuity is made more noticeable because of the drastic difference in hairstyles and dresses worn by the two actresses.
When Don and Kathy are in the car together there is no windshield, but when she pulls up to R.F. Simpson's house there is a windshield. And when she leaves his house the windshield is noticebly larger.
When the diction coach is reading "Moses Supposes", Cosmo is making faces behind his back. When the coach catches him in the act, they both flinch. It then cuts to a wider shot and they both flinch again.
During "Make 'Em Laugh", the green couch slowly changes position. It is in front of the hallway backdrop when Cosmo first enters that part of the scene, but it has moved stage left by the time Cosmo does the back flip off the backdrop.
During "Beautiful Girl" number, the camera shows all the outfits curving downwards, starting with the pajamas and ending with the wedding dress. But when the camera closes in on the man singing and the women coming behind him, the wedding dress is in the middle with the others going outwards.
About three-quarters into the film, Don Lockwood is with Kathy inside a movie studio showing her features of the background sky and lighting, and he throws on the light switch. He then comes to the area of the big fan where the top blades are in a 10 o'clock and 2 o'clock position. Soon after, he returns to the fan area and the blade positions are different. They then change back to the original position as he again returns to switch on the fan to blow on Kathy.
In "Make 'Em Laugh", when Cosmo sits down on the couch with the mannequin, his hat is pulled down over his forehead and the brim is flat. In the close-up, however, his hat is pushed back and the brim tilted up.
Near the end of Don Lockwood's silent film "The Royal Rascal", his character leaps over the railing and falls as he lands on what looks like a patch of soft sand. Then it cuts to him getting to his feet, but the sand is gone and he is on what appears to be brick or flagstone paving. Also, the area immediately below the railing is now filled with tall plants where in the previous shot it was a small pond where he threw one of his attackers.
Before the song "Moses Supposes", Cosmo is standing behind the diction teacher and pulling funny faces. When the teacher turns and catches Cosmo pulling a face, Cosmo straightens his face to a normal expression. The camera then zooms out and we see that Cosmo is still pulling the funny face and the teacher catches him in the act all over again.
During the fight scene in "The Royal Rascal", Don's character pushes the guard with the spear over the railing on the stairway. As the guard falls, the hands of a crew member are seen reaching out to grab his legs.
It shows that every studio started making Talkies after The Jazz Singer was released, but even the major studio balked at the idea. At the most, studios would release two Talkies a year, but they still released them also as Silents, since most cinemas were not equipped for Talkie films. Talkies were believed to just be a Fad. The last Silent film was made in 1936, ten years after The Jazz Singer came out.
Near the beginning of the film, when Don Lockwood flees from his fans outside the Chinese Theater, he jumps via other vehicles to the top of a tramcar. Running along its roof, he holds on to the trolley pole (current collector). This is live, and he would be instantly electrocuted.
The same crowd cheering noise is used twice in the opening scene outside Grauman's Chinese Theatre - first after Don Lockwood says "Not in front of all these people!", and secondly after Dora Bailey presses Don Lockwood to tell his life story.
As the curtain opens on the preview showing of "The Dueling Cavalier", the title card shown on the screen spells it "Duelling". However, since both spellings are in fact correct (from a grammar perspective), it is entirely possible that one was used during production, and the other adopted only at the preview.
R.F. Simpson plays a talking picture demo film on a screen at the after-party for "The Royal Rascal." The demo film on the screen appears to be a matte shot because the top of Simpson's head is cut off for a moment as he walks in front of the screen just as the projector illuminates the screen before the film starts.
At the end when Kathy will sing "Singin' in the Rain" for Lina, Lina is asked "What key should the song be played in." She says "A-flat". The band leader says, "Singin' in the Rain in A-flat", and then proceeds to conduct the song in E-flat.