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 standing in that same spot, not moving, for over a year.
(at around 16 mins) The orange juice on William's shirt disappears after the kiss in the kitchen.
(at around 7 mins) 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.
(at around 12 mins) 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, it is later seen that the bathroom is only two flights up (at around 1h 9 mins). 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.)
(at around 1h 35 mins) When William is listening to Anna at Kenwood House, he is wearing his glasses. When he stands up and gives his earphone back to Harry, the soundman, his glasses are gone.
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. At one point, she is seen about to leave the house in her wheelchair, unassisted.
(at around 1h 50 mins) Spike manages to let himself out of the boot of Max's Peugeot 406 estate car when he gets out to stop the traffic, but in reality there wouldn't be any handle to open the liftgate from the inside.
(at around 11 mins) 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.
The book shop is a thousand yards away from William's house. It would be quite a walk just to buy a juice there (since he left the shop and was close to his house when bumping into her).
(at around 37 mins) At Honey's party, the lamp and photographs behind Bernie's head, when he is talking to Anna about what she does for a living, are not in the same place after camera shots from him to her.
(at around 1 min) In the opening montage of Anna (before William and Anna meet), the shot of her getting out of the limousine is from the scene at the end, when William is actually with her.
(at around 1h 40 mins) Towards the end, when Anna gives William the painting, over William's right shoulder is an orange coloured book called "Comics, Comix and Graphic Novels," which is not, in fact, a travel book.
Sparrow chirping is clearly heard in multiple scenes through the film. In fact, by the time the film is set, sparrows have mostly disappeared from London.
(at around 12 mins) Just after Anna and William have collided in the street, the camera operator can be seen reflected in Anna's sunglasses.
(at around 20 mins) Anyone familiar with Notting Hill will know that the rooftop terrace scenes with Spike and William and later with William and Anna (at around 1h 11 mins), 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.
(at around 11 mins) After William leaves his shop to get the orange juice (which he subsequently spills over Anna), it is revealed in conversation that he is walking in the direction of his house, which is 'just over the road'. As both the travel shop and William's house are close to Portobello road, he was walking in the opposite direction back to work.
(at around 58 mins) William leaves the Ritz Hotel to return to Notting Hill but boards a night service bus N52 on the opposite side of the road going to Piccadilly Circus.
(at around 45 mins) 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 (at around 1h 6 mins).
(at around 21 mins) All Anna knows about William after their first meeting is that he lives in a house with a blue door. Yet somehow she manages to ring him up and leave a message with his flatmate.
(at around 52 mins) When William confronts the crude men in the restaurant, the mic can be seen above the men's table briefly.
(at around 11 mins) After Anna and William collide in the street, the microphone shadow is visible on the left side of the screen (widescreen version).
(at around 18 mins) 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".
(at around 39 mins) During Honey's birthday party, Anna tells Bella she is a vegetarian. However when confronting the loud men in the restaurant (at around 53 mins), she tells them, "The tuna's really good."
(at around 1h 13 mins) 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 from Hugh Grant's. Looking from Anna's perspective, it can seen that Grant's "smaller" toes are all smaller than his big toe; some of the stand-in's "smaller" toes are longer than his big toe.
(at around 47 mins) At Honey's birthday party it is revealed that William's nickname is "Floppy." After Anna climbs the fence into the private garden she says "Come on, Flopsy."