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; only the first two episodes remain, in dodgy copies made with an experimental technology (the later two serials are preserved in full, though in Quatermass II the quality is still inferior).
The Other Darrin: Each of the five serialsnote 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.
Accidentally Accurate: 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.
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 to a fiery Scot. Astonishingly, it works.