When Kat and Nick get on the boat, Kat's hair is mussed but as soon as Kat takes off her shirt her hair is perfectly in place. Clearly if it were mussed before, taking her shirt off should not make it better.
On the night Kat comes back and has her fun with Nick and they proceed to move to the boat in the driveway, they are clearly seen standing and walking around inside. From the outside, the boat is obviously not large enough for that to happen.
Not only are there no virgin flights arriving at 4.20pm (as pointed out by another poster) but even if there were, it would have to leave New York in the early hours of the morning but it is clearly daytime when Kat goes to the airport.
When they first walk into the drinks reception Debra's coat is flowing open revealing the red dress she has finally chosen to wear after changing at the pub. They walk in and meet Aunt Bee but when they turn around her coat is fully buttoned and tied at the waist.
When Kat first arrives at the engagement party and her mother hugs her, the strand of hair (artfully) hanging on the left side of her face gets pushed back and stuck on the side of her head...but as the camera cuts back and forth, sometimes it appears in its original position and sometimes 'stuck'.
When driving to the bachelorette party, the car Kat and Nick are driving is on the left side of the road. In the next shot it is still on the left side of the road, but the car's license plate is backwards.
In the scene where Amy asks Kat to pass her the drink, Amy plants a big kiss on Edward's lips. When they fall down and get up, we can see a visible red lipstick stain on Edward's lips. In the following shots it disappears.
On the wedding day, when Nick picks up Edward from chasing Jeffrey, his big flower is fresh and wide open. Then it is all wilted and brownish. The flower also ranges from fresh and open to wilted-looking during the wedding ceremony. It's also wilted at the reception.
In the scene were Kat is talking with her stepfather just before the wedding, when he first approaches her, to the left of Kat, a door is shut. When it shows her getting up to leave to go and get Nick, the door is wide open.
When Kat is bringing her bags to the airport, she has three hard case powder blue suitcases and two soft ones, piled on top. When she is leaving the airport after landing, the cart has her three hard cases, one powder blue soft case, one of his black soft cases and he is carrying his other black soft case. Kat is carrying nothing. When they get to the house, all five pieces of her powder blue luggage can be seen again.
When Kat and Nick arrive at the house in London where they spend their first few nights, it is broad daylight outside. It is still daylight as Nick showers and they discuss the magazine article of his that Kat has brought. Then, in the following scene, it's suddenly dark and they are making and crawling into the bed, with no hint of them having gone down to dinner or done anything else in between during which a sufficient amount of time would have passed to explain this change from day to night.
When Kat and Nick are approaching London, there is an announcement being made indicating the final decent into London. The woman on the PA says that the local time in London is 4:22 PM. Virgin Atlantic does not have flights from New York to London that arrive in the afternoon, only late at night or early morning.
When Kat and Nick stop at the pub to get dressed, you see Kat standing on a paper towel in the bathroom (for obvious sanitary reasons). Yet, whenever she comes out to display an outfit, she is barefoot.