Dawson Casting: A minor case. In the extended edition, Lois Lane is the young girl the teenage Clark waves to as he's running past her train, implying that Lois Lane is a number of years younger than Clark Kent. Margot Kidder, however, is four years older than her co-star Christopher Reeve.
Slight reversal in the case of Reeve. Although Superman only says "Over 21" when asked about his age, it's later stated that he arrived on Earth in 1948 after a 3-year journey (consistent with apparent age of young Clark as he came out of the spaceship). Assuming the movie takes place circa 1978, the year it was released, that puts Superman in at least his early 30s, but Reeve was only in his mid-20s.
Deleted Scene: Included in some foreign cinema releases and TV airings before the DVD release included them all.
An unusual case where no deleted scene(s) exist, but the editing of the movie makes it appear as if there is. Luther and his cronies are shown discussing the idea of traveling to Africa to obtain a piece of Superman's home planet. The scene ends with them preparing for their trip. The very next scene takes place in a mountainous area of the United States where suddenly Luther and his gang execute their hijack of the nuclear missile. The sudden narrative jump to this day has people wondering if there wasn't an entire sequence of events between the two sequences - i.e. the Addis Ababa caper, or at least a transitional scene involving Superman - that was filmed and cut.
Actually, the film seemed to suggest that Luther and Company didn't go to Addis Ababa after all, and that Luther stole the kryptonite from the Metropolis Museum - remember when Perry White was giving Clark a lecture on how he should be more aggressive, he mentions that the Museum was broken into and two security guards were killed - but all that was stolen was a piece of meteorite...
Doing It for the Art: Richard Donner made "verisimilitude" his mission for the movies. He wanted it to feel real, which was no easy feat. There's a reason why "you'll believe a man can fly" is the tagline, though.
Real-Life Relative: Steve Kahan (credited here as Stephen Kahan) played Artemus, the undercover cop who had the misfortune to lose his partner thanks to Luthor. Kahan, Richard Donner's cousin, is better known as Captain Murphy from Donner's Lethal Weapon series.
Executive Meddling: In 1940, Jerry Siegal wrote a story called "The K-Metal from Krypton," that would have introduced an early version of kryptonite and Superman revealing his secret identity to Lois. Siegal's publishers nixed this story as damaging to the franchise and made it clear the Status Quo Is God that hold for years while both story innovations would have to wait for years.
Tribute to Fido: The miniseries A Superman for All Seasons, by Jeph Loeb and Tim Sale, gave teenaged Clark Kent a dog named Shelby, after Sale's own dog. It was a two panel gag, but Shelby later became more notable as the golden retriever in Smallville.
A man by Walter George Alton revealed in an interview that he was considered for the role of Superman but ultimately turned down for being too old. Mystery Science Theater 3000 fans may recognize him as the star of Puma Man.