Breakthrough Hit: The film was a critical and commercial success and established George Lucas as a film powerhouse. However, most people know him more for the later Star Wars movies instead (despite Lucas not expecting it to be better-known).
Dawson Casting: Charles Martin Smith (18) and Ron Howard (18) were the only two real teenage principal actors of the film. Most of the remaining principal cast members were in their 20s with the exceptions of the 12-year-old Mackenzie Phillips, and Harrison Ford, who turned 30 during filming.
Executive Meddling / Re-Cut: Universal trimmed four minutes from the film for its initial release. When it was re-released to theaters in 1978, following the success of Star Wars and Lucas' resulting clout, the cut scenes (including Toad dealing with a fast-talking car salesman and Bob Falfa serenading Laurie with "Some Enchanted Evening") were put back in. Producer Coppola openly sympathized with Lucas over the matter, as he himself also dealt with executive meddling while shooting The Godfather.
Universal thought so little of the film (not knowing how to market it, and certain that as it had no stars it would flop), that it sat on the shelf for six months before the studio finally decided to release it. To their great surprise, it became enormously successful at the box-office.
Serendipity Writes the Plot: Bob Falfa wears a cowboy hat because Harrison Ford refused to get a period-appropriate haircut and consequently be sidelined from working in other projects while waiting for it to grow back out.
Toad crashing his moped into the trash can during the opening credits was an actual, unscripted accident.
When John and Carol are sitting at the red light, a car full of girls pulls up next to them. One of the girls throws a water balloon through the window and it hits Carol. It was scripted to hit the side window and drench Phillips' face, who was then supposed to act really angry. However, she was accidentally hit square in the face and unable to refrain from laughing. Still, she kept going, ad-libbed through the scene and George Lucas kept it, as he did with many presumably garbled first takes in this movie.
When the guy robbing the liquor store tosses the bottle to Toad, he almost misses it. According to Charles Martin Smith, they had practiced the scene almost flawlessly, but Lucas decided to keep the one take that was the mistake.
Wolfman Jack's line, "Sticky little mothers, ain't they," when shaking Richard Dreyfuss's hand, was improvised.
The scene after the drag race in which John admits to Terry that he was losing when Falfa's car lost control and rolled was improvised by Paul Le Mat and Charles Martin Smith. They had not had time to prepare for that scene, as it had been scheduled to be shot at another time.
Troubled Production: Although the shoot finished on time and on budget, it was no small miracle that it managed to do so:
The day before shooting was due to begin, a key crew member was arrested for growing marijuana, and setting the cameras up for location shooting on the first day took so long that they did not start shooting until 2am, putting them half a night behind before a single scene had been shot.
After a single night of outdoor filming in San Rafael, the city revoked their filming permit after a local bar owner complained that the road closures were costing him business, forcing them to move filming twenty miles away to Petaluma. On the second night, a local restaurant caught fire, and the noise of the fire engine sirens and the resulting traffic jams made filming impossible.
Inevitably for a film featuring so many driving scenes, the cars and equipment required to film them in motion seldom behaved as planned. An assistant cameraman was run over after he fell off the back of the camera truck during filming of a road scene, while filming of the climactic drag race was hampered when one of the cars broke an axle, then broke the replacement axle, and then nearly ran over two cameramen lying in the road to film its approach.
Among non-technical problems, Paul Le Mat had to be rushed to hospital after suffering a walnut allergy flare-up, Richard Dreyfuss had his forehead gashed after Le Mat threw him into a swimming pool the day before his closeups were to be filmed, Le Mat, Ford, and Bo Hopkins were claimed (according to a biography on Lucas with no source citations) to be drunk most nights and every weekend, and had conducted climbing competitions to the top of the local Holiday Inn sign and one actor set fire to Lucas' motel room.
And when the film was screened for a test audience, Universal Studios representative Ned Tanen told Lucas the film was unreleaseable, prompting an outraged Francis Ford Coppola (the film's producer) to offer to buy the film from Universal and release it himself while Lucas, burned out from the chaotic film shoot, could only watch in shock. Instead, Universal offered a compromise whereby they could suggest modifications to the film before release. It was not until 1978, after the success of Star Wars, that Lucas was able to re-edit and release the film as he originally intended.
Apparently, Mark Hamill auditioned for a role in this movie.
Budgetary reasons meant that George Lucas to drop the opening scene, in which the Blonde Angel, Curt's image of the perfect woman, drives through an empty drive-in cinema in her Ford Thunderbird, her transparency revealing she does not exist.
Lucas wanted a few Elvis Presley songs, but with the soundtrack being absurdly expensive right from the start, they were the first on the list to go.
If the Star Wars Expanded Universe novel "Alien Exodus" had been published, it would've been Canon Welding of the highest order- a descendant of Curtis Henderson, Dale Hender, and a couple of allies would've escaped Earth as it was being taken over by computers (leading to what occurred in THX 1138- Hender was to be the first of the THX series), only to fall into a hyperspace disturbance, into the Star Wars galaxy and onto what would eventually become Corellia (briefly facing a beat from Willow as well). A further story then detailed how Dale's descendant, Cosmo Hender, eventually became the first of the Skywalkers.