Before Louie takes his gorilla costume off when he talks to Rex, the face of the costume is under his hat on top of his head.
When the ghoul actor puts his mask back, his eyes are hidden behind those of his mask, when he's about to crash on Louis and Cecilia, the mask's eyes are gone, revealing the actor's eyes.
When the ghoul land on its platform, two trails of smoke coming from under his and his partner's robes are visible. When the ghoul jump off from its platform to scare the audience, the smoke trails are gone.
The Macy's Parade is going in the wrong direction.
There's a major error with regards to the Triceratops and the Parasaurolophus (Dweeb) in this movie. Both of these dinosaurs were herbivores, plant eaters, yet when they first appear in the film, they're eating hot dogs, which are essentially meat.
Rex's three claws on his forearm are anatomically incorrect for a Tyrannosaurus Rex. He should only have two. Many older movies, such as in Disney's Fantasia and the original King Kong, make the same mistake, as intact T. Rex forearms had not been discovered. This changed in 1989, when complete forearm specimens began to enter the fossil record, though the theory of two claws had been around for decades. This could just be part of Rex being anthropomorphized for animation.
The dinosaurs land in New York City on Thanksgiving Day, but most of the residents (including a main character) are wearing spring or summer clothing. Thanksgiving is in the middle of autumn, meaning the residents of New York would be bundled up.
When Stubbs tells Louis and Cecilia to wave their Pitchforks, this is the only moment where Louis is actually seen with a pitchfork.
The Eccentric Circus' main stage alternate between a pentagram and a more generic design at different parts.
When one of the punk teenagers taunts Louie, Cecilia cries out in fear but her mouth doesn't move.
It seems strange that Buster (the little bird Rex tells his story to) would even question the fact that he is interacting with an animal intelligent enough to speak and play golf (albeit one believed to be a member of an extinct reptilian species), considering he and his family also somehow have the ability to speak. While Rex explains very early on how his own humanistic abilities came to be - by consuming a cereal that is designed and later shown to increase intelligence, personality and overall development in animals - those of the little bird remain unexplained throughout the entire film. Was some of that cereal added to the bird feeders in Central Park offscreen? There's no additional explanation as to how Rex can communicate with another animal that otherwise is not known to have the same abilities.
It has never been revealed how or why Professor Screweyes uses real ghosts and dragons to scare people while the Demons and Ghouls are actually actors in costumes.
Someone mistakenly describes Dweeb as an Apatosaurus. He is in fact a Parasaurolophus.