The Airplane tickets/boarding passes?
This may have been brought up before (probably a lot of things have, given the age of the movie). Anyway, wouldn't they notice at the airport that they had an extra ticket/boarding pass? Wouldn't their names be on the tickets, not to mention passports being checked?!? How did they not notice that Kevin wasn't there???
'No offense' means I'm about to insult you, but don't get mad.