Cast the Runner-Up: Ian Colin replaced Keith Pyott in The Quatermass Experiment; Pyott ended up with a more minor role.
Disowned Adaptation: Reflecting on the 1979 serial, Nigel Kneale said, "Frankly, I was never happy with the whole idea in the first place. The central idea was too ordinary". Although Kneale was pleased with the high production values, he was dissatisfied with the casting, believing that John Mills "didn't have the authority for Quatermass". He was similarly unimpressed with Simon MacCorkindale, noting that "We had him in Beasts playing an idiot and he was very good at that". Kneale disliked the depiction of the Planet People, as his inspiration had been angry punks rather than hippies (as evidenced by his portrayal of Kickalong as a gun-toting lunatic who commits multiple motiveless murders).
Missing Episode: Most of the TV version of The Quatermass Experiment is missing, because it was performed live in the days before any reliable method of recording television programs existed. The BBC initially planned to record the entire run for sale to the CBC in Canada, but quit after the first two episodes due to the dodgy quality of the telerecordings (a fly can be seen on the screen during part of the second episode!)
The Other Darrin: Each of the five serials (the four TV serials, and the radio serial) features a different actor as Quatermass. The original actor died, and every subsequent actor proved unavailable after completing one installment.
Real Song Theme Tune: Gustav Holst's "Mars: Bringer of War" from The Planets is the theme for the first two serials and the 2005 remake.
Throw It In!: The famous shock at the climax of part three of Quatermass and the Pit when one of the "dead" Martians appears to lurch at its discoverers from the just-opened forward section of the capsule, was not in fact planned by director Rudolph Cartier but was the result of a lucky accident when the Martian prop slipped down from its position unexpectedly, resulting in giving the creature the semblance of movement and causing the actors - and audiences at home - to draw back in fright.
Accidentally Correct Writing: In Quatermass and the Pit (1967), the protagonists uncover remains of primitive humans from five million years ago. The characters state that no such remains have ever been found back that far in time before. In 1974, Lucy would be found and she would be the oldest human/hominid remains at 3.2 million years until even earlier specimens were found, making the concept of humans being in existence five million years ago well within possibility.
Executive Meddling: American Brian Donlevy was cast as Quatermass in the first two films so they could be sold easier in the American market.
Market-Based Title: The films were retitled The Creeping Unknown, Enemy from Space, and Five Million Years to Earth on US release, since Quatermass wasn't a selling point outside the UK.
No Stunt Double: Julian Glover had to perform his own stunts for Quatermass and the Pit, including the scene where Colonel Breen falls over into the pit.
The Other Darrin: Not quite as bad as the serials, but the length of time between the two fifties films and 1967's Quatermass and the Pit meant Brian Donlevy was replaced by Andrew Keir. In other words, Quatermass changes from a scowling American in his mid-fifties to a fiery Scot of barely forty. Astonishingly, it works. (Even more astonishingly, when compared with the TV version Quatermass changes from a plummy-voiced Englishman to a Scotsman, speaking some of the same dialogue.)