On roof of the hotel Mavis is looking at the sun with her eyes in shadow.
When Dracula takes Johnny up to the hotel room with Glen's trapdoor, there's no shrunken head on the door. When he escorts Mavis out, there's now a shrunken head there. This head also has the same voice as the head on Mavis' door.
In Quasimodo's Kitchen Johnny frees himself from the gag to speak and its around his neck. But after Drac rescues him the gag disappears.
When she is in the roof, Mavis can't see the sunrise because she's in the shadow. The chimney covers the sun.
(at around 45 mins) When Mavis and Jonathan are on the roof, watching the sun rise, the lower portion of the roof becomes lit by the sun first, leaving the upper portion in shadow. A real sunrise would light the roof from the top down.
When they are playing Bingo the announcer says N27. In actual Bingo, only numbers 31 through 45 fall under the N column. Twenty seven would actually fall under the I column.
After Dracula & Jonathan have a heart to heart ("...Someone scratch me. I have fleas...") the front of the hotel is shown as though illuminated by the Sun. However, the Sun is behind a mountain (behind the hotel) and the facade of the building should have been in shadow. Compare the dark right hand wall of the castle and the right hand (dark) face of the smaller building before the castle.
Griffin makes a comment about seeing a human when they enter the village in Transylvania , but aside from Quasimodo who has a hunch back , Griffin though invisible is human .
In the first chaos scene after Johnny enters the hotel (when he realizes the monsters are real) a fly is depicted with multiple eyes (like a fly would have). In all later depictions of the flies, they have human-like eyes, with whites and pupils, etc. However, it could be that they are different types of fly monsters.
On the roof Jonathan exclaims "Look at the view from here, you can almost see Budapest..." The film is set in Romania, so perhaps he meant Bucharest, more likely to be spotted from a high point in Transylvania. However, Budapest (capital of Hungary) is not much further away from Transylvania, and as Transylvania used to be part of Hungary (more precisely the Austro-Hungarian Empire) at the time the original Stoker novel was written, Jonathan might indeed mean Budapest.
Beethoven was deaf in real life, so he couldn't respond to Dracula when he is a zombie.
(at around 1h 19 mins) After Dracula chases the plane with Johnny on it, he crashes into Mavis' room and returns to his human form. When he appears through the smoke, his face is sunburned and there are holes burnt in his cape. In the very next shot, his skin is its usual pale white color and his cape is intact. What actually happens is that his cape, clothes and skin can be seen repairing themselves.
Wayne the Werewolf is seen in a sauna. Dogs, members of the wolf family, would have died in severe temperatures meaning that Wayne would have too.
As Dracula heads up to Mavis' room at the beginning of the movie, he passes by a number of other hotel rooms. The last door he passes, the witch/maid greets him, and the head says, "Maid, clean up this room!" You can see the number plate on this door, room #174. Later, after Mavis' "trip", Dracula approaches her door, and we can see the number plate, room #174.
While Eunice is playing Bingo, and has five cards infront of her with different dot patterns, all the cards have exactly the same numbers in all the same positions. When the shot pans to the old monster next to her, she also has the exact same bingo card.
Drac is able to both float in the air and fly , but yet when he and Johnny are fooling around in the ballroom , he nearly falls to the floor before Johnny catches him , why didn't he just turn into a bat or float to the next table .
Drac tells table 31 to hit table 19 even though Johnny is clearly on table 36 as that was the number he called out when he jumped on the table and started to float around the room .
There is a suit of armor standing near the door of the ball room when Quasimodo talks about making human pot pie after they find Johnny , so why did it not inform the other suits of armor about a human being on the premises especially since the hotel was supposed to be human free .
Griffin comes very close to hitting a sheep in the road , but as he swerves to avoid doing so , the next shot shows them a little farther back and not really close to the sheep at all .
The mannequin in the window goes from being an actual mannequin to being a zombie portraying one , back to a regular mannequin .
A pair of red shoes and what appear to be pink slippers go from being on the top shelf of Mavis's closet to sitting on the floor just outside of it .
When Drac crashes through the window in Mavis's room , you hear the glass breaking even though the window is open , and when he stands up and walks towards Mavis you can see he is sunburned and his clothes and cape are damaged , but in the next shot , not only is the window closed and in one piece , the damage to his clothes and cape are gone and they look brand new and his sunburn is gone as well .
Drac somehow manages to get out of the car at the monster festival without crawling over or behind Griffin .
Dracula is unable to hypnotize Johnny who wears <1mm thin plastic contact lenses but successfully hypnotizes a plane pilot through a 15 mm thick reinforced, insulated and coated compound plastic and glass cockpit window.
Dracula is able to erase mind. When Quasi found out Johnny was human, Drac could have erased his mind so he wouldn't tell the other monsters later.
Despite being totally isolated for the better part of 120 years, the monsters are able to know what a Whoopie cushion and Doritos are.
Dracula, Frankenstein's monster, The Mummy and The Invisible Man are in the car in the village of Transylvania. However, when Dracula speaks to a human dressed as Dracula, the glasses of the Invisible Man are not seen.
Dracula being a vampire should have burnt to a crisp long before he was ever able to reach the plane .
Frankenstein, despite being terrified of a tiny match and unable to fly because an airplane engine might catch fire, is okay with a torch-lit castle and a sauna powered by a fire-breathing dragon.
Unlike many characters in the movie that were once human but later became monsters, Quasimodo (aka the Hunchback of Notre Dame) was never a monster but simply a human with a physical deformity.
When playing bingo, after the number is called, every player stamps in the incorrect column except for Mavis.
Dracula was able to freeze Johnny in mid-air while he's falling. But in Quasimodo's Kitchen he froze Quasimodo but Quasi fell after being frozen.
The Character of The Invisible Man is being called Griffin in parts 2 3 and 4, in the first part they call him invisible man while in the credits he's credited as Griffin.