At the beginning of the film, Mary and Roger's nanny Brenda refers to Mary as "Mrs. Anderson", but according to the credits, Mary and Roger's last name is Charleston
As Brian is being picked up by his wife, Carol's bags can be seen on the front steps. Carol had already been picked up by a driver prior to Brian's departure.
Sarah says the line "How does she take her makeup off? With a chisel?" as if "With a chisel?" is an exclamation, not a sarcastic question.
The film is set in 1992, by which time British Rail had long since stopped steam rail services. Either Peter's house happens to be on a heritage line, or a steam train was added to make the UK appear pleasingly old-fashioned for US audiences.