When Hugo says to the automaton 'Is this your card?' (at around 33 mins), the card is in his right hand but in the next shot it's in his left hand.
After Tabard finishes showing A Trip to the Moon (1902), the reel is still full. When Georges Méliès enters the room the reel is empty.
During one scene in the book shop, Isabelle is clearly seen walking in front of Hugo, who stops behind for a moment. Immediately in the next shot, Hugo is walking in front of Isabelle.
When Inspector Dasté answer the phone from the bathtub, he answers it with his right hand all wet and soapy and the telephone is at his right side, next scene he's holding the phone with his left hand, the phone is behind him and both of his hands are completely dry.
When Hugo (in his dream) sees the key on the tracks, it is in the gravel next to the cross tie, when he jumps down to retrieve it it is laying in the middle of the cross tie.
The old Montparnasse train station where the action takes place did not have a clock tower. The clock shown in the movie is instead reminiscent of the clock at another Paris train station, the Gare d'Orsay.
Georges Méliès tells Hugo to fix a wind-up mouse. After Hugo finishes, Georges winds the toy mouse and places it in front of Hugo and himself. After scurrying around the table, the mouse stops in front of Georges, turns toward him and sits up. Then it moves in front of Hugo and sits up again after turning to face Hugo. The mouse is a toy and could not have performed those precise actions.
Most of the many Georges Méliès excerpts seen in the film were made prior to 1910. Their accompanying piano music is the song "By the Waters of the Minnetonka" by Thurlow Lieurance, first published in 1913. But as these were silent films, and so would not have contained soundtracks, whatever music accompanied a screening of such a film would either be performed live or played from, say, a phonograph. So it is perfectly reasonable for this film to portray a older silent movie being screened with slightly newer music.
When Hugo finds the key on the tracks in his dream, it is partly buried in the stones. In the next shot the key is on the sleeper (wood plank connecting the rails). But, since this is a dream sequence, continuity can easily change.
When the Automaton falls onto the tracks, the train has to brake really fast to avoid it, but clearly the end of the track is 15 feet further down the track. So the train should not have been moving very fast in the first place. This was an intentional mistake; the scene was based on an actual derailment at this station, the same one featured in Hugo's dream, that was caused by a train coming into the station at too great a speed to stop in time.
When the automaton is drawing the image, it begins by dipping the pen in an inkwell, and the nib emerges with black ink clearly seen on it. However, subsequent closeups of the pen show the nib dry, and a black pencil lead can be seen beneath the nib, which is what actually creates the marks on the paper.
When Hugo and Isabelle talk in the street outside her apartment, they are shivering and it is snowing but we cannot see their breath, revealing the scene was probably shot in the studio.
When Georges Méliès winds up the toy mouse after Hugo fixes it, there are distinctive color changes in the pixels of the counter space beneath it to the right and bottom right of the mouse's path, revealing an editing clean-up gaffe.
The movie is set in 1931. From 1925 to 1934 the Eiffel Tower had illuminated signs for Citroën that adorned three of the tower's four sides. However, in the movie the lights on the tower are as they are today, with no Citroën sign on it.
The accordionist in the film is shown with a piano accordion, but Parisian accordionists in those days almost exclusively played the chromatic button accordion, and most still do.
When Hugo and Isabelle are watching Safety Last! (1923), the music of the movie is a soundtrack written in 1990.
Until 14 July 1989 the Eiffel Tower was lit by spotlights from the outside. That night, in a spectacular fireworks display, the outside lights were turned off and the interior lights were turned on.
In the two scenes with a close-up on the tracks, you can see the rail fixed to cross-ties by Pandrol clips, instead of traditional bolts. Pandrol clips were used for the first time in 1957.
The guitar player's hand movement mismatches the sound of guitar in every scene he's seen in.
Since 1889, the Eiffel Tower has been the tallest structure in Paris. Yet when Hugo and Isabelle are at the top of the clock tower at the station, the camera clearly looks DOWN at the top of the tower (from 1:20:00 to 1:20:02 and 1:20:47 to 1:20:53).
The Montparnasse Train Station where most of the action is supposed to take place is alternatively shown on the same side of the river as the Eiffel Tower, and on the other side. It actually is on the same side.
When Hugo and Isabelle are standing on a bridge, Notre Dame Cathedral is behind her to the west. The camera shifts to Hugo and he points behind himself (east) a mile away to the Montparnasse train station where he lives. The Montparnasse station is actually nearly two miles southwest of Notre Dame.
In the opening sequence in the station, couples are dancing in front of a restaurant slate board advertising "Plats du jour" (suggestions of the day). Among these is "Boeuf bourgignon". The correct spelling should be "bourguignon".
The story is set in Paris, yet the characters all have English accents.
Despite the story taking place in a train station which could be considered a somewhat international place, all the characters speak English all the time, yet the movie is set In Paris, France.