Kate McCallister: What kind of idiots do you have working here?
Desk Clerk: The finest in New York.
In the first film, it is mentioned several times that Kevin is 8 years old. However, in this film it is stated that he is 10. When his parents are in the Miami airport security office, they say that he was left alone last Christmas. That would make him 9. Both films take place the same days, from about three days before Christmas until Christmas morning, so even if Kevin is born around Christmas time this is still a mistake.
When Kevin arrives at the New York Airport and he asks the woman at the counter which city he's in. That woman is Ally Sheedy. This film was written, and produced by John Hughes, who also directed her in the 1985 film, "The Breakfast Club."