Continuity mistake: The woman at the gate to the New York flight arranges the tickets in a neat pile with the edges in line. The next shot, the tickets are a mess again.
Other mistake: While Kevin asks the ticket agent in New York what city he's in, there are signs at the gate indicating departing New York LaGuardia and destination Los Angeles. No gate at any airport would have a "departing" city sign - only destination city. Travelers know what city and airport they are physically at and leaving from.
Revealing mistake: This film takes place around Christmas, however, when the airport shuttle vans leave the McAllister house to head for the airport, there are short shadows under the vehicles, indicating it was filmed during the summer.
Plot hole: There is absolutely no way that a hotel staff employee would ever let a child check in alone. The staff would be required to personally contact the owner of the credit card Kevin hands them personally to make a reservation, not to mention Kevin never had his dad's government I.D on hand, so there was no proof of verification whatsoever, which is also required. Not possible today or even back then, especially considering the lawsuits, terminations, and bad press that it could lead too.
Other mistake: The old man that receives Kevin's bag in Florida can be seen walking through the airport when the family runs past him. The man could not have boarded the flight to Miami, as the family ran past him and were the last to board. The man was wearing shorts when he received the bag (but he could have changed on the flight in theory - had he been able to board). An old woman with him was in the same situation. (00:12:52 - 00:17:36)
Answer: I think they are just more shocked with how brazen Kevin is with telling off his uncle.
Phaneron ★