The character of Duchess is very different in the second scene (the will) than in the rest of the film. In particular she has her nose more clearly defined. The rest of the scene seems also to have been designed by different animators, who used a more "sketchy" style.
When Scat Cat and his gang are playing "Everybody Wants To Be A Cat", one of the cats is playing the guitar left-handed, and a moment later right-handed.
As the crate with Edgar in it slides through the barn door, the padlock suddenly appears on its latch.
In the country lane shot right before we first meet Napoleon and Lafayette, Edgar's motorcycle's side car switches sides. This is due to the series of cells being photographed backwards while editing.
The studs on Duchess' collar disappear and reappear throughout the movie.
When Edgar first hears that the cats were the primary inheritors, he begins to plot their demise. He imagines millions of dollars, with "$" signs dancing in his eyes. But since the movie was set in France, the money unit should have been francs.
During Edgar's motorbike chase he passes a signpost showing directions to Tour Eiffel and Pl(ace) de L'etoile spelled in the French, but 'Park' Monceau which in French would be 'Parc' Monceau.
In the beginning of the movie, Edgar is driving the carriage from the left of the bridge to the right of the bridge on the way home. Later in the movie, when Edgar is driving his motorcycle with the cats in the sidecar, he travels from left to right on the bridge but going away from home. However, the Eiffel Tower is visible in the first scene but not the second, so the bridge may be being viewed from the opposite direction.
In the opening scene when Edgar helps Madame off the carriage, Madame feeds Frou Frou, but no food is visible in her hand.
When Marie, Berlioz, and Toulouse race each other to their little cat door, they fight over who will go first through the door. When Marie mentions that she's a lady and should go first, it can be seen through the door that it is nighttime. After Toulouse comments she's not a lady, and Berlioz pulling her tail saying she's nothing but a sister, it is once again daytime as seen through the door.
When Thomas O'Malley is singing to Duchess, in one shot her collar is completely blue.
In the beginning credits, they show that the song She Never Felt Alone by Robert B. Sherman and Richard M. Sherman is in the movie - but the song never played in the film. The first part was supposed to be played before the lawyer came into the bedroom that the will was created at by the old woman, and the second part was supposed to play when Duchess explaining to Thomas why she has to return to Paris.
When singing, "Everybody Wants to be a Cat," Shun Gon mentions fortune cookies. The film takes place in Paris, France in 1910. Fortune cookies were first baked in San Francisco in 1918, eight years after the events of the film.
Traffic lights are shown during Edgar's drive through Paris with the cats. Apart from a traffic light installed in 1868 (which exploded in 1869) in London outside the Houses of Parliament, traffic lights were first used in 1914 in Cleveland, Ohio, four years after this movie takes place. (with three-color traffic lights first used in Detroit in 1920.)
The milk van that the cats catch a ride on is a Fordson BB, which didn't go in to production until the late 1920's, and the film takes place in 1910.
The truck at the end of the film is a Morris-Commercial FE Series, which wasn't made until 1955, which was 45 years after the film is set, and 15 years before the film was released.
In the scene where Edgar is ironing his clothes, he is wearing striped boxer shorts, where this kind of underclothing wasn't invented until 1925 - 15 years after the setting of this movie.
When Berlioz leans on the piano keyboard right before practicing, two notes are heard, which are D and G-sharp, which are three steps apart. However, the keys he are leaning on are A and B, which are only one step apart and not the notes that were heard.
When Marie hops up onto the piano, the lowest keys are hit, but the sounds come from keys in a much higher register. The same thing happens when Berlioz slides down the keyboard to get back at Marie for pulling him off the bench; the keys sound much higher than they should.
The label on the trunk says "Timbuktu, French Equatorial Africa", but Timbuktu (now in Mali) was in French West Africa (specifically the colony of Upper Senegal and Niger), not French Equatorial Africa.
Why does Edgar think he needs to get rid of the cats in the first place? They can't use money, so he'll effectively be in charge of it anyway. Not only that, he seems to overestimate the lifespan of each cat, basing his calculations on their "nine lives." Most cats only live 10-15 years, meaning that Edgar would inherit the money within a relatively short time.
Why is Edgar thinking the inheritance is happening right away, to the point that he has to hurry to get rid of the cats? Madame Adelaide is not dying.
Near the end, after George notices the music coming from elsewhere in the house, the scene changes to the alley cats playing their instruments and dancing. Scat Cat is two places at once--on the top of the piano with other cats (third from the right), and also on the floor in the bottom right of the screen.
When Thomas O'Malley tells Roquefort to fetch Scat Cat and the alley cats, he doesn't tell him where to go.
After Edgar is kicked into the box that he was planning to use to send the cats to Timbuktu, it is shown with the padlock once again locked and sealed on the front, despite the fact that Roquefort had removed the padlock and there had been no time for anyone to put it back on before it was kicked outside.
Edgar refers to the money that he would inherit as "dollars" and the American dollar sign flashes in his eyes; however, the film takes place in France.
When the Alley Cats fight Edgar, Roquefort orders everyone to be quiet while he works on cracking the padlock. Even Edgar stops. This is strange since Madame Adelaide Bonfamille and the other humans can't understand animals. It's especially strange since Edgar can't understand his horse.