When Kate leaves the wedding and is standing in front of the elevator, there are 2 rooms between the elevator and the room Harvey goes into to play the piano. When he starts to play, the camera picks up a shot of her peeking into the room where Harvey is playing while she is still standing at the elevator. This is impossible at the room Harvey goes into is clearly down the hall from the elevator, not the room next to the elevator.
Harvey and Kate have met up on the train...in one scene (from the front) she is holding a book. In the next scene (shot from the back) she is fiddling with her scarf. In the scene after that (from the front once again) she is holding the book.
After Harvey gets back from taking the business phone call at the dinner the length of his tie is wrong, the little end sticking out from under the thicker end. When is flashes back to Harvey the tie is fixed.
Harvey arrives at the restaurant with an electronic tag still attached to the sleeve of his jacket, as seen during the taxi scene. Inside the restaurant he is introduced to his daughter's friends but he doesn't shake hands as he doesn't want the people to see the tag, but after a while there is a shot where the sleeve doesn't have the tag anymore.
Susan's wedding reception was in the Grosvenor House Hotel on Park Lane in London on the 14th floor according to the elevator when Kate was leaving the party. There are only eight floors in the Grosvenor House Hotel.
Harvey and Kate arrive at Paddington Station then walk across much of London to the South Bank. They then appear to the east of Festival Hall, though crossing the river from Charing Cross they would approach it from the west.
During the wedding ceremony scene, Harvey's daughter is wearing a very casual beige dress, but later at the reception she is wearing a formal beaded wedding gown with ornaments in her hair. However, this is a common practice in British weddings: for each event the dress gets more formal.
The day Harvey met Kate, it's autumn everywhere. When she tells him to go to the wedding reception, he asks her the time and she says it's 7pm however it is still daylight. This is unusual for London where sunset is around half past 6 in the autumn. At the end of the movie, when they are walking away on the south bank, the tree leaves are fully green.