Continuity mistake: When Conrad first puts the spider lock in his pocket it is put in his left pocket, but when he takes it out, he takes it out of his right pocket.
Continuity mistake: At the end of the movie when the Cat finished cleaning up the mess and was packing Thing 1 and Thing 2, everything looks immaculate except in one scene, in the corner of the living room, there are still purple splashes.
Continuity mistake: When the children hear the bump coming from upstairs, they go to investigate. Conrad says "it sounded like it was coming from the closet." The closet is facing the stairs when they go up them. Later in the film, a shot is shown where you can see the rooms in the whole house together. The closet isn't on this shot, so where has it gone?
Continuity mistake: In the scene when the Cat is with the kids in his car, right after they show his new steering wheel being a chain linked one, the next shot shows it as a regular steering wheel, and then back to a chain linked one for the rest of the scene.
Continuity mistake: When the cat is going into the living room, there are at least 6 pillows on the couch but when he says he's gonna take a look under the hood, all the pillows are gone.
Continuity mistake: In the scene where the cat is a piñata, the actual piñata has no opening showing the candy. Later when the cat is screaming Conrad sees the piñata with a huge opening on the belly where all the candy are showing.
Continuity mistake: When the Cat is juggling all the items in the beginning, he has a cake on his hat. After he burps and everything flies up he catches everything but the cake is now gone and never appears again.
Continuity mistake: In the scene where the cat's tail accidentally flips Sally onto the couch,we see that the couch is very far away from where the cat is, yet after a 2 to 3 second shot of a CD player coming out of the cat's hat, we see Sally standing behind a chair right in front of the cat, this is impossible as no human can move this fast.
Continuity mistake: The first time Lawrence comes in (as the trio are jumping on the couch), he starts sneezing while standing under the Cat. He's in the middle of the living room and facing the front entry; but in the next shot, he's on the edge of the living room and facing in. He jumps back and forth throughout the scene.
Continuity mistake: When Sally, Conrad and the Cat are getting on Mrs. Kwan, they are facing a tunnel with a table and two chairs above it and a water slide behind them. When they go through the tunnel they end up coming out the same tunnel, with a table and two chairs above it, before they go up the same slide Mrs. Kwan first came down, except they didn't show the toilet at the start - it was a tree. (01:00:15)