While Anna and William are sitting in his living room and he is reading a newspaper, she notices the size of his feet. It is clear that there is a stand-in for Will in the shots looking at Anna because his toes are different than Hugh Grant's. When you are looking from Anna's perspective, you can see that Hugh Grant's "smaller" toes are all smaller than his big toe. The stand-in's "smaller" toes are longer than his big toe.
As William first enters the elevator at the Ritz, he asks the other person what floor they're going to. They respond "three", but when they both leave the elevator, the number "1" is clearly visible on the elevator's floor indicator.
Three separate times during the movie, the same mother and child are seen in the alley beside William's book shop. All three times this person and child are wearing the same clothes and are in the same physical position. According to the chronology of the film, they would have been stood in that same spot, not moving, for over a year.
Honey starts the film with shoulder length hair. In the "Ain't No Sunshine" montage, Honey's hair is cut short, understandable in the passage of time, but in a subsequent scene in the book shop when she gives Will Anna's agents' phone numbers, her hair is long again. When she announces she's getting engaged to Spike at the closing of Tony's restaurant, it is short again. The film only spans about a year.
When Anna first comes into William's house, William tells her, "The bathroom's on the top floor, and the telephone's just... just up there" (pointing to the landing, one flight up). However, we see later that the bathroom's only two flights up; William's bedroom is on the top floor, three flights up. (While there's a phone in the kitchen, he presumably wants to let her have some privacy.)
When William leaves Hampstead Heath after overhearing Anna talking to the other actor on set, he is wearing a dark blue shirt and has no jacket. The next shot has him entering the book shop with a light blue shirt and a jacket slung over his shoulder. (The next scene, which is set the next day, has him wearing the pink shirt for the remainder of the film.)
In the first scene where William encounters Anna, Rufus (the book thief) is shown several times in the background. Initially he has several days growth of beard, and in one of the shots he appears to be freshly shaven, then back to the growth of beard.
When Anna wants to use the bathroom, it is the door seen from the bottom of the stairs. Later on that same day it becomes William's bedroom door. Also, in one shot, the bathroom door has windows. Later in the movie, it becomes a plan wooden door.
Near the end of the movie, Anna Scott goes to the store to talk to William Thacker. When he comes out of his office and stands in front of a desk, behind him is a stack of books, all the same style. They are all facing the same way. The next scene the top two are facing another way from the bottom one.
When William Thacker visits the set at Hampstead Heath he is stopped by a security guard at a checkpoint. When he enters the set after speaking with Anna Scott not only is the guard gone but so is any type of security checkpoint.
When Anna first comes into William's house and William tells her the way to the bathroom, the blackboard beside William says in big letters "SPIKE CLEAN UP". In the next shot from behind William the letters on the blackboard are much smaller.
Will has just spilled orange juice all over Anna's shirt and is telling her that she could go change at his house. The left side of his face is fully bathed in sunlight in shots from the front and is in the shadows in shots from behind.
When William takes Anna back to his flat, after the orange juice incident. His shirt is stained, she comes into the flat and changes. Then she leaves. When she returns to get her books, his shirt is still stained but after she kisses him the stain is gone. An impressive kiss...
When you see the entrance area of Will's bookstore from inside for the second time the shelf is shifted to the left, and the things things on it are rearranged, e.g. the latitude scale of the left globe is turned.
When Bernie and Anna are discussing their incomes in the dinner scene, there's a lamp and some photographs on a table over his right shoulder. Both the lamp and the photos change position as the view changes back and forth between Anna and Bernie.
At the bookstore, in the wide shots the surveillance monitor is sitting in the center of four large books, yet in the close-up of the monitor, showing Rufus, the shoplifter, there is another book lying on the larger books, to the left of the monitor.
When Anna visits Will's place for the first time there is a plant and an open window when you look from the entrance into the apartment. Later, after she has left, there is no plant and a shutter pulled down.
In the scene where they are all having a farewell dinner at Tony's restaurant, first the spaghetti is in the large bowl with Tony mixing, then suddenly the spaghetti has been dished to to each person's individual bowls.
In the scene just after Anna opens the door and all the photographers are outside, Spike walks to the front door in his grey underwear. Just before he opens the door the underwear is covering only one butt cheek (his left). The next shot outside the house the underwear is adjusted and covers both butt cheeks but then when he enters the house and is checking himself out in the mirror, his butt cheek is exposed again.
In the scene where Julia Robert's character first goes to Hugh Grant's character's flat, there's a chalkboard that's telling Spike to clean up. It's quite small writing, but in a close-up of Hugh Grant when he's directing her upstairs it's clearly larger. It then goes small again until she comes downstairs, when it goes large and stays that way.
After William leaves the movie set, in the next shot he goes back to the book shop. Seen through the window is the table with the books and globes. Then when Anna is standing near the window, we see that there are completely different kinds of books on the top shelf now.
On the surveillance monitor there are lying two books, the upper one with a black binding. In a close-up of the monitor you see only a part of the book with the black binding, and the other book is gone.
When Hugh character takes Julia Robert's character back to his flat, after the orange juice incident. She isn't wearing her sunglasses in the next shot she leaves the building and she is wearing her sunglasses again.
In the scene just before Anna goes into the bookshop with the painting there is an outside shot of the building. In front of the school you can see the woman kneeling down in front of a child but also if you look through the window and door of the shop you can see Will walking in wearing a blue shirt. In the very next scene he is sitting at his desk wearing a pink shirt.
In the 3-shot sequence where Honey is opening her birthday gifts, the shots show the following: 1. Hugh Grant puts a necklace on Honey and she is wearing the hat; 2. She has just opened the hat and is not wearing the necklace; and 3. She is wearing the hat and no necklace. In the following sequence (the competition for the brownie), she is wearing the necklace and no hat.
When William initially walks into his bookstore after he takes the mail from Martin, on the bookcase he approaches to his right we see close-up some of the books on three shelves, one of which says 'Siberia', and also the two small posters, the bottom says 'Mukiwa' and the top says 'Lonely Planet', tacked to the side of this bookcase. When Anna browses that same bookcase, she looks at the book 'My Turkish Delights'. The books that are on the shelves of the bookcase, and only the top poster that is tacked to the side are now completely different.
Anyone familiar with Notting Hill will know that the rooftop terrace scenes with Spike and William and later with William and Anna, were not remotely in the same neighborhood as William's flat with the infamous Blue Door. The rooftop scenes were filmed most likely on rooftop between Ladbroke Grove and Stanley Gardens, while the flat with the Blue Door is located on Westbourne Park Road.
After William leaves his shop to get the orange juice (which he subsequently throws over Anna), it is revealed in conversation that he is walking in the direction of his house, which is 'just over the road'. Given the location of both the travel shop and William's house both on, and close to Portobello road, he was walking in the opposite direction back to work.
When William and Anna leave the birthday party, they turn right, and William points that way when he suggests Anna come to his house. However, William turns left out of the house to go home after the "perfect girl" date and subsequently spending the night at Max and Bella's house.
An addled customer enters William's travel bookstore and asks for books by Charles Dickens. He's sent away with an admonished, "This is a travel bookstore", but a true travel book professional would know that Dickens wrote several notable travel books, including "American Notes" and "Pictures from Italy".
In the private garden, both when Will and Anna climb the iron railings and also at the end of the film, the park bench has obviously just been placed there. Normally there would be no grass in the front of the bench where people put their feet when they are sitting down since people's feet trample and finally kill the grass. In the film, there is plenty of grass in front of the bench.
Although Bella is in a wheelchair and complains of the fact that her house needs ramps for her to get around, there is a step without a ramp leading to her front door. In one scene she is seen about to leave the house in her wheelchair, unassisted.
When William bumps into Anna with the orange juice, his arm is slightly stretched out. It would be hard to hit Anna, soak her clothes with orange juice and get such a load of the fluid on his own shirt.
In the scene where they are all having a farewell dinner at Tony's restaurant. At the end of the meal they are all scattered around the restaurant, the camera is on Bella and Honey and in the background, outside the restaurant, 4 extras walk past, 3 at the front and one behind. The next shot is at the window where Max and William are sat, again the 4 extras walk past in exactly the same order and positions. They are the same people because one of them has a pink top on and you see that twice.
In the scene where Will and his friends are driving to catch Anna at the press conference, Spike jumps out of the back of the car to halt traffic and let their car through. When he closes the back door, the sound effect of a slamming door is loud, but the door doesn't shut - you can see it bounce back up.