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Incorrectly regarded as goofs
When the Baroness attempts to kill Cruella by burning down her flat, she has realized Cruella and Estella are one in the same, and later questions John about not killing her only child. Yet she's surprised to learn Cruella is her child when Cruella brings it up. However, she was clearly only pretending to learn of it for the first time to fool Cruella into believing that this information would actually make a difference in her behavior.
After Cruella lights up her dress and is brought to the Baroness, her arm switches places during her conversation.
When Cruella shows up after Estella is killed the car slides on the rocks and leaves tire tracks but in the next scene there are no tracks.
Cruella crashes a car into an old-fashioned British red phone box, which buckles on impact. These phone boxes were made of cast iron which would, with sufficient force, break, not bend.
Quite early in the film Cruella, Jasper, and Horace are shown picking the pockets of passengers standing in the aisle of the upper deck of a London double-decker bus. Standing has never been allowed on the upper deck.
Anita's camera is a 1972 Canon Canonet GIII QL17. The attached flash, a Canolite D, would not be suitable for professional use due to the long warm-up time between shots.
When the metal detector is being used to find the necklace in the dog poo, it makes quite a bit of noise but the operator continues searching as if there's nothing to stop for.
It's established that Cruella's black and white hair is the source of bullying at school, but there's never any suggestion that she dye it one color, despite her mother's pleading with her to fit in with the other students. However, even as a kid Estella was very attached to her unique hair and wasn't about to change it, especially since she never had any real desire to fit in to begin with.
In 1964, Cruella, Jasper, and Horace are shown running across a zebra (black and white pedestrian) crossing. Zig-zag markings, indicating a no parking/no overtaking area, are on the road either side of the crossing. These markings did not come into use until 1971.
In the vintage clothing 'Second Time Around' shop scene, Artie points out a Chanel from the spring 1950 collection on his racks. Chanel had closed down in 1940 and didn't reopen until 1954, so there was no spring 1950 collection.
The jail scene shows numerous prisoners behind bars. Even in the 1970s, police cells in the UK did not have bars. UK cells often use steel doors with a very small window.
While Cruella and The Baroness where riding in the back of her limo and pulled into The Baroness mansion, the car that was following her was a 2000s range Rover.
After the dumpster truck crashes into the police station, it reverses out and a beeping noise is heard. Whilst back-up beepers were invented in 1963, they were not widely used in the UK until the late 1980s.
Cruella steals a motorcycle and is shown heading south across Waterloo Bridge. However, when the view switches to a close-up, the National Theatre is visible in the background, which means she is still on the bridge but is now heading north.
The UK doesn't have Police Commissioners (except for the Metropolitan Police largely associated with London). Hellman Hall is clearly coastal, and the Metropolitan Police would not be responsible for anywhere far away enough with huge cliffs.
When Jasper and Horace are checking the Dalmatian poo to see if the necklace had been expelled, they use a metal detector. One of the issues they discuss is not knowing which dog ate the necklace. They could easily have used the metal detector to resolve this problem.
Although Emma Stone is thought to speak Received Pronunciation fairly well, she still uses American pronunciation for vowels such as in "don't" and "walk."
"Grift" is an Americanism; in the UK, they are referred to as "cons" and "scams."
The UK doesn't have raccoons despite these being mentioned in one of Cruella and co.'s scams as a feasible pest invasion.