You Look Familiar: Susan Saint James played an SIA agent under cover as a flight attendant in the original pilot ("Magnificent Thief"). She then made her first appearance as "Charlie" Brown in the series first episode, "It Takes One to Know One". While they're clearly different characters ( the SIA agent gets murdered), you'd think Al would ask Brown, "You have a twin sister?"
In the pilot, Wally Cox is briefly seen as a man trying to buy an airline ticket. In the series' final episode ("Project 'X'"), an Episode on a Plane, Cox plays airline passenger Professor Moses.