When Chloe and James first go to the Peter Pan statue, there is no bench, but a later scene shows them sitting on one.
When everyone is "escaping" from the fake hostage situation, they use the air duct system. However, the air duct system magically becomes a laundry chute, and they all end up in a pile of laundry.
In the beginning of the movie, when Dylan is introducing all of the members of the Model UN team, he says they are from "Fairview High". A few seconds later the team is introduced as being from "Lakeview High".
In the first scene at the Peter Pan statue, it has started raining, and both James and Chloe suddenly have umbrellas, though neither is shown carrying one in any of the scenes from earlier in the day.
When Riley (Ashley Olsen) is shopping, her hair is completely straight. Then when she is trying on the dress her hair is wavy. When she is walking with Brian, after trying on the dress, her hair is straight again.
In one scene from one of the hotel rooms in London a can of Diet-Coke is seen. In Great Britain that drink is sold under the name Coca-cola light.
The girls torment a guardsman outside the railings of Buckingham Palace but the Guards were moved inside the forecourt a generation ago.
Lay's Potato Chips are not sold in the UK - the equivalent brand is Walker's.
When the taxi driver says that they are passing Clarence House where the Queen Mother lives, they are in fact passing Horse Guards Parade.
When Chloe, Riley, Dylan, Brian and Rachel are playing a game of catch to test each other on their knowledge of England, Chloe incorrectly gives the Queen's official title as "Her Royal Highness" (HRH) instead of "Her Majesty" (HM) - and neither her sister nor any of her friends correct her. In the British monarchy the style of HRH is associated with the rank of prince or princess, the Queen is HM which ranks above HRH.