When Indiana and his father are on the blimp they later sit at a table talking about Dr. Henry as a father. Notice several times when Indiana moves away from the table or even when he only sits there that he has no pants on. In fact you can see his bare legs and at one point his black briefs. At the beginning of the scene while they are at the table you can even see Indiana's pants! I heard that it was hot when they were filming and Harrison Ford just got too warm...