Continuity mistake: When Conrad, Sally and the Cat come out of the bedroom after they have first met, the camera zooms on Sally. The door behind her is closed. Goes to wide shot-Conrad is still walking out the doorway and the door is open. After that, the door remains closed.
Continuity mistake: The pipe on the side of the house that Lawrence came out of as the house went back to normal, doesn't exist at any other time in the movie.
Continuity mistake: In the wide shot of the countryside, when Joan and Lawrence are going to Joan's house, you hear the police siren, but you don't see the police vehicle. Suddenly, there are no other cars, and there's the Things posing as motorcycle cops right behind Joan.
Other mistake: When the Cat is using the periscope to look at Nevins, the range on it reads between forty-nine point two meters, and fifty-two point four meters. These distances translate into just over one hundred sixty-one, and one hundred seventy-one feet respectively. The kids and the Cat were no more than about fifty to sixty feet at best away from Nevins.
Suggested correction: Its more plausible the device isn't calibrated properly, after all it's not like they're going to lob something that requires pin point accuracy such as a bomb for example. That and the fact the cat is keen to make a huge mess at every opportunity suggests he probably made a mess of calibrating the periscope too.
Continuity mistake: Towards the end of the FUN song, Cat has a bottle of milk on a silver tray balanced on his foot. When the camera is looking at Sally, the bottle on the tray comes up into view, but now, there's a glass of milk on it that wasn't there before.
Continuity mistake: When Lawrence comes in and sees Conrad and Sally jumping on the couch, he goes into the kitchen to nick all the food. He has a sandwich in this right hand, but after he starts sneezing to leave the house, the sandwich is on the left hand.
Continuity mistake: In the scene where the cat cuts off his tail, when he first hits it with the knife it bounces off but in the next shot it went through.
Continuity mistake: In the beginning of the movie the polka dots on Joan's dress are big, then when the cat in the hat takes the dress out of the closet to clean the mess on the walls from the cupcake batter exploding from the oven, they're smaller.
