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 Larry takes out his teeth when going to sit on his chair just before the repo men come and take his TV, he puts his false teeth in a glass. Immediately after that you can see his top teeth still in.
Continuity mistake: When Cat fills up the cupcake tray it is very messy with cupcake batter all over the tray but when he closes the tray it is clean and perfect with the batter in each cupcake holder perfectly.
Continuity mistake: Right after the Cat, Sally, and Conrad crash, Sally's hair bow is half way down her pig tail. In the next shot, it's back on top in a perfect bow.
Continuity mistake: When thing one and thing two come and they start messing everything up, they get the cupcake mix from the previous scene on Sally and Conrad, but in the next shot it's gone.
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: When the kids and Cat are in the S.L.O.W driving toward the red light, the gears and wheels in front of the windshield are not turning, and the rest of the time, they do.
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: After Cat reads the three plans to Conrad and Sally, the "Plans Folder" disappears from Cat's hand after they run away from their hiding spot.
Continuity mistake: When the Cat sees Conrad and Sally's mom his hat gets all scrunched up, but in the next shot his hat is straight again.
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: After the cupcakes explode out of the oven you see the purple goo splash the dog all of its face and left side. However, the next time you see the dog there's hardly any purple goo on him, just a little bit on its face.
Continuity mistake: In the scene where Conrad and Sally are signing the contract, they have the pens in their hands, but when Cat says, "Give me five!" the pens have disappeared.
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)
Continuity mistake: After Conrad tosses the candy to distract the kids, the one that hit Cat in the groin is in the foreground to get some candy. After a quick cut scene, he's standing triumphantly behind the other kids with his arms raised.
Chosen answer: Theodor Seuss Geisel, or Dr. Suess as we know him, published most of his books between the late 1930's and the late 1980's. "The Cat in the Hat" was first published in 1957. Dr. Seuss' works generally tell the stories of fantastical characters in imaginary places, meant to be timeless. Illustrations and animated adaptations show buildings and objects with unusual proportions, odd shapes and bizarre functions. The live action film of "The Cat in the Hat, " however, is rooted to reality by its decidedly human child protagonists in an ordinary house in an ordinary neighborhood. The production design, costume design and set decoration of the 2003 film seem also to have the goal of achieving a certain timelessness. No date reference is given. However, there a decidedly stylized quality of 1950's-1960's suburban architecture and design, complete with its generic forms, chimneys, picket fences, and colors such as yellows and avocado greens, reflecting the common decor of the time. Similar to the 1971 TV short, which seems to provide a reference point for the design aesthetic of the film, nothing appears exceedingly futuristic nor rooted in period styles like victorian or colonial. I have also posed your question to Rita Ryack, the film's costume designer, whom I found on Facebook. If she sees my questions and decides to respond, I will add her insights to this answer.