Revealing mistake: When Anna and Hans are dancing in front of the lighthouse, we don't see the shadow of the railing or the ground they're standing on.
Continuity mistake: When Tom the Minion dressed as a maid is hoovering, he answers the front door by opening it towards him as does the minion who looks out of the door after Tom has been taken. In the scene with Gru sat on the steps in the rain, sad that Lucy is off to Australia, Agnes comes out with an umbrella, but now the door opens outwards.
Continuity mistake: In the scene where Grug is chasing after Guy, they both wind up stuck in a tar pit. After successfully escaping they are covered in tar and Grug even gets himself stuck to Guy. In the next scene everyone is running from the exploding ground up between the mountains and both Grug and Guy are completely clean and free of tar.
Continuity mistake: Lotte barges in the pizza place and carves her name onto Shaggy and Scooby with her sharp nails. LOTT on Shaggy's shirt, and the E in Scooby's fur. They run out the door and then come back to pay for their food. Suddenly the rips in Shaggy's shirt and the scraped off fur on Scooby's chest have vanished. (00:23:45)
Other mistake: Sam draws an orange vest for Lockwood using a dark crayon. (01:10:45)
Continuity mistake: In the hotel, at Gagamel's room when Clumsy suddenly opens the food lid and bonks the room service guy. The lid falls at the right side of the door, but in the next shot, when Gagamel opens the door, the lids is on the left side. (00:55:55)
Factual error: In the scene where the young Helen Goff is traveling by train with her family, it's obvious that an American engine and coaches have been used, instead of a Queensland Rail locomotive of the era. None of the QR locomotives had flared smokestacks like on the one seen in the movie. The coaches are also incorrect - as far as I know, none of the Queensland Rail wood coaches had clerestory roofs with a set of windows in them like seen in the movie. When Helen looks out the back of the train, it's also obvious that the tracks are spaced at 'standard gauge' (4' 8.5"). They should be closer together, as railways in Queensland are built to 3'6" gauge instead. There is also no such company as the 'Queensland Victoria Railway Co', as marked on the coaches, as all railways in Australia are state owned.
Continuity mistake: When Percy is tossed though the air by the Cyclops the fleece disappears from Percy's hand, but reappears when he lands on the table. (01:18:25)
Other mistake: Towards the beginning of the film, the teacher counts 19 students then says one is missing, implying there are 20 (including Mike) However, Mike ends up being paired with the teacher, despite there being an even number of students and therefore everyone should have a partner.
Continuity mistake: Towards the beginning of the movie, Gerry's son spills hot coffee all over Gerry's shirt. He is wearing a white shirt/vest for his uniform. There is no stain or even wetness left behind.
Continuity mistake: When Dusty first takes off for the big race his "D7" on the right side of his tail disappears. (00:32:35)
Continuity mistake: After the failed attempt to drop a mace on Mr. Ginormous' head, the giant grabs Jack, Tom, and Jerry and taunts them. He then throws them onto the table and Jack's hat comes off and flies away from him. Shot changes angle to Jack trying to sit up and pop off at Ginormous and suddenly his hat is right next to him. (00:43:15)
Continuity mistake: In the scene where Percy is talking to Stephen, Thomas reverses down the track he has arrived on and stops. But when Percy leaves, Thomas has mysteriously disappeared. Also, the track Thomas was on ends at some buffers behind him and leads to the turntable in front of him, but that was set to Percy's line.
Continuity mistake: As in most movies I've seen where size is a main factor, the animators did not pay much attention to scale. In the scene where the main character leads the leaf people to her father's house, she is jumping and lands on the thinnest pencils I've ever seen. They have the width of the pencil smaller than her foot. However later on, it is shown that a push pin is almost the size of her body.
Continuity mistake: The Appraiser puts the condemned tape across Dorothy's wrecked porch. The tape from the bottom right to the upper left is the one in the front of the other diagonal line, but then it's the other way around. (00:07:30)
Continuity mistake: During the first meeting between Sebastian and Belle, the dog drops the game far back on the hill, but the rest of the scene takes place with the dead hare right in front of the 'beast', on the edge of the rock. (00:10:10)
Continuity mistake: When Billy pulled the medallion out of the ground, part of the cloth covering it was hanging past his hand, but when he blew dirt off the cloth, the cloth was compactly covering the medallion. Also, the cloth should have had more than some loose dirt on it and the medallion - after buried in the ground for 200 years - should have been dirty and tarnished, not clean. (A cloth covering the medallion would not protect it from soaking rains). (00:23:35)
Continuity mistake: During the first show, when Mariah Mundi puts the bag over Bizmillar's head, the position of the bag and Bizmillar's hands changes between shots. (00:44:56)