When Jim flies to Seattle, the plane shown taking off is a DC-8. The plane shown flying at altitude is a 747 painted only in Boeing's house livery, not that of a commercial airline. The airplane shown landing is a 747 painted in American Airlines livery.
The establishment shot of the SEA-TAC airport, showing the digital clock in front of the terminal with the sounds of planes landings, is a still shot, the cars in the background never move.
All the vehicles involved in the car crash scene are from the late 50's/early 60's. Probably stock footage.
In the black-and-white film at the end, Jim and John Micelli are shown drinking from what appear to be aluminum cans, but the first aluminum beer can didn't appear until the late 1950s and the film was made during the Korean War (which concluded in 1953).
When Jim is calling the air charter service from Jennifer's apartment in Seattle, the telephone number shown on her phone is (311) 555-2368, the very same number shown each week as Jim's number during the opening credits of the show.
The vehicles in background of some of the Seattle street scenes have California license plates. When all the vehicles have Washington license plates, these scenes are filmed at a studio.
When Jim parks in front of the dentist's office, he passes up the last open parking space and parks against the red curb.
In one of the very early episodes Rockford borrows a friends old white Mercury station wagon. In this episode when he finds Jennifer leaving the bank she goes and gets a car and it appears to be the same old white Mercury station wagon from the earlier episode.
(at around 11 mins) The mic enters from the bottom of the screen, reaching as far as Jim's knee, where it continues to wobble about for 12 seconds.