SPOILER: After the credits Neal's boss is still at his desk analyzing the ads, his Thanksgiving dinner sitting on his desk.
The scenes shot at Lambert Airport in St. Louis were shot during winter but the weather was uncharacteristically warm (mid 80's F), so all the snow in the scene had to be trucked in.
Del Griffith's friend at the railroad, Bert Dingman, is a direct reference to Robert O. Dingman Jr., President of the New York and Lake Erie Railroad where the train scenes were filmed.
In the airport scene in Withita, when the airline employee announces that the flight has been cancelled, you can see on the board behind him that the destination of the flight is "nowhere".
The exterior of their aircraft in flight is a reuse of the 707 flying through the storm from the movie Airplane! (1980).
John Candy and Steve Martin eat dinner on the plane in a scene that is not in the theatrical version though it airs on the televised version. The scene ends with a long-haired passenger in front of Steve Martin letting his or her hair cascade down onto his brownie, completely covering it. No longer hungry, John Candy fishes through the hair to retrieve and eat it.
The green convertible is a 1986 Dodge 600 ES Turbo, with a 2.2 Turbo engine, it was modified for the film. The manual can be seen in the glove compartment when Neil puts his wallet in there.
John Hughes shot over 600,000 feet of film, almost twice the industry average. The rumored three-hour version of the film does indeed exist, although not in order - moreover it's a mess of footage that would take "months, maybe even years" according to Hughes to transform into an actual film. It is locked away in a Paramount vault, and according to Hughes, most of it has probably deteriorated by now.
John Hughes' original choice for the train station and platform was the station in Kankakee, Illinois, 60 miles south of Chicago. The cast and crew were in town for a week waiting for weather cold enough to make snow... and several interior scenes were filmed at an abandoned warehouse using a "cover set".
Although it's not included in the theatrical or the network cuts, a shot of Del Griffith brushing his teeth was included on ads for the network version.
In a television version of the film, there is added footage during the New York-Chicago/Wichita flight where Del and Neal try to eat in flight.
Neal's house was also a set built from scratch, consisting of seven rooms and taking five months to complete. It ended up costing $100,000, which angered Paramount executives and caused turmoil on the set.
No transportation company wanted to appear inept or deficient in any way, so crews had to rent twenty miles of train track and refurbish old railroad cars, construct a set that looked like an airline terminal, design a rent-a-car company logo and uniforms, and rent 250 cars for the infamous Rent-a-Car sequence.
Cast and crew traveled from the Midwest to the East Coast and back in search of snow for many scenes, which seemed to melt whenever they arrived. The shoot was hellish, and according to some who worked on it, Hughes's grumpy behavior (he was going through rough times) only made it worse.
The Marathon Car Rental scene is exactly one minute long from the time Steve Martin starts his tirade, to the time the attendant ends the scene. In that 60 seconds, the word "fucking" is used 18 times.