Continuity mistake: When Andy and Miranda are in the limo in Paris, they pull up to the curb and there is a bunch of press waiting as Miranda opens the door. However, when the shot changes, it is not the same group of people - in particular, there is a balding photographer who should be standing right there but is nowhere to be seen. It all happens too quickly for that much change.
Continuity mistake: In the opening scene, Andy is seen eating a bagel, then the shot changes and she has no bagel, but when she goes down the stairs into the subway the bagel is back in her hand.
Continuity mistake: In the scene where Andy is sitting in the restaurant with her father, you see her father taking off his glasses. In the next shot you see the father from the back, taking off his glasses again.
Continuity mistake: When Andy throws her sidekick into the fountain, she is holding it in one hand. The reverse shot of the face of the sidekick shows her holding it with both hands.
Continuity mistake: When Miranda puts her coat and bag on Emily's desk in the scene where Andy has to tell Emily she isn't going to Paris, the bag is standing up. The next shot shows the bag flat on the desk and turned.
Continuity mistake: In the scene in Paris where Christian kisses Andy, from one angle her head is tilted to the right as they kiss, then the shot changes and they are now tilted in the opposite direction. This happens almost every time the angle changes throughout the kiss.
Continuity mistake: When she sings the "Happy Birthday" song to her boyfriend she carries a little cake with a burning candle on top. The length of the candle changes from view to view and appears longer, shorter and longer again.
Continuity mistake: When Nigel is toasting himself about what he thinks will be his new job, the glass is in his hand. The next shot shows his hands are empty and he grabs his head with both hands and walks to the window. Andy then brings him the glass. The glass is in his hands and the very next shot (no time lapse) shows the glass on the counter and then next (no time lapse) in her hands.
Continuity mistake: This one has to be watched carefully but can be seen without slow motion - when Emily hits the taxi's windshield, the front shot shows her hit the hood with an orange package under her - it looks more like a box than a bag - and her purse in her right hand. Before she was hit she also had a small orange bag in her hand. A shot later, the side view of the accident shows 3 orange bags flying through the air, and her purse is on her left arm, not on the right as it should be.
Continuity mistake: When Andy is talking to Nigel at the fashion shoot she tucks her hair behind her ear and when the shot changes it's completely over her ear.
Continuity mistake: When Andie's talking to Emily in the hospital, Andie puts her purse down. When they cut back to her again, she puts it down again.
Continuity mistake: When Andy meets her friends in the restaurant, Lily is holding a french fry which goes from short to longer, from straight to crooked, over the course of three or four shots.
Continuity mistake: When Miranda tells Andie that she wants the Harry Potter book, Andie goes back to her desk and her Sidekick is on her desk. She had put her purse down but not her phone because she never got a chance to pull it out.
Continuity mistake: In the restaurant where Andy is giving her friends presents, she hands her friend a little clear case of Clinique products, but when she's putting them in her friend's hand, she is also holding a blue box when she wasn't before.
Continuity mistake: When Andy is talking to her boyfriend while doing the science project, she picks up the bottle of beer while getting up to go and hug him. The bottle disappears when the scene cuts to the hug.
Suggested correction: Given that how the shots change, it's likely that she would have put it on the desk again.
Continuity mistake: Emily has a bad cold/flu the day of the benefit Miranda hosts - her nose is extremely red and she's congested. Miranda tells her Andy will have to go too, so Emily shows Andy photos of clients to memorize; suddenly she has no sign of a cold at all - the redness just disappears, which wouldn't happen that quickly. That night it's back in full force, just as it was earlier in the day.
Continuity mistake: At her first day of work, Andy is telling Emily about Miranda's demands, and as Emily asks "Did she say which skirts?", her arms are on her hips, but as the shot changes, her arms are folded in front of her, with no time for the shift.
Continuity mistake: Not only does Andy not have the bagel when she enters the subway stairs, but she is wearing her gloves/mittens when she does - when the bagel reappears, they are gone.
Continuity mistake: In the scene right after Andy finds out about Miranda's divorce proceedings, she goes back to her hotel room to get ready for her date with Christian. When she looks at her reflection in the mirror as she's applying mascara, her hair is a little tousled and tucked behind her ears. In the very next shot, her hair is untucked and perfectly in place.
Continuity mistake: When Andy sees Miranda on the street at the end of the movie, Miranda gets into a Mercedes then tells the driver to go. When the car takes off, Miranda is in the car but as the Mercedes drives away, Miranda is no longer in the car.
