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.
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.
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.)
You Look Familiar: Sladden the drill technician in Quatermass and the Pit is played by Duncan Lamont, who played Victor Carroon in the original BBC serial.