When Izzy is going on behalf of her defense she doesn't have the fluffy pen. The next shot of her shows her with the pen.
When Tiffany is trying to mess with Annie, it sounds as if she's pronouncing her name "Andy" when calling her over, but if you listen closely she's actually calling her Izzie, as part of her plan that relies on her pretending that she can't tell the twins apart.
At the end, when Tiffany's boyfriend inserts the password and username in the PC of the teacher, you can see, that the username is hidden by asterisks and the password is shown in clear.
At the beginning of the movie, when the girls are still in England, it is raining, and they complain that the sun never shines there. However, the sun is clearly shining brightly even through the rain.
During the "driving on the wrong side of the road" joke, all of the shots inside the car and the establishing shot of the car clearly show the vehicle is being driven on the right side of the road as it should be (i.e. street sign facings, other cars in the shot mirroring their driving path)
When Izzy is locked in the bathroom, she tries to escape and at one point stands on the toilet and her foot slips inside the bowl, and it flushes, despite her hands being nowhere near the handle and her foot falling inside the toilet and not touching the handle either.
The equipment van is visible from behind the car in the "driving on the wrong side of the road" gag during interior shots.
During multiple instances in the movie when Annabelle and Isabelle are quoting the rules to the headmistress, there is an obvious distinction in where they are looking in the book and where the headmistress is looking in the book. Usually Annie and Izzy are looking somewhere in the beginning while the headmistress is looking somewhere in the middle.
The Headmistress scolds the twins when they add extra accents to their school uniforms by reading them the dress code rules, which states that shirts must be monochromatic in color. Monochromatic refers to a color scheme composed of one singular color, all the kids shirts are either the original blue or pink for the twins, so all shirts were monochromatic and complied with the rules.