When Gwyllim looks out of his bedroom window, the village below is clearly a still, as emphasised by the two static pedestrians lower right of frame.
The movements made by Wilt Morgan when he plays the concertina do not match the music we hear.
The 2 police motorcycles do not have "Police" (nor "Heddlu" in Welsh) marked on the front of them.
Although the Welsh lead character is named Gwyllm, at least some video copies have subtitles calling him Wilhelm.
The 2 uniformed patrolling motorcycle police in Wales (United Kingdom) are shown to be routinely carrying guns. Northern Ireland is the only part of the United Kingdom where uniformed patrolling police routinely carry guns - in Wales, England, and Scotland they don't.
When the doctor (Edward Mulhare) is thrown back down the hallway by a glance from Griffiths (David McCallum), you can see the cable connection on the back of the stunt man as he rolls over at the end of the stunt and before the scene is cut.
The names of the characters (Evans, Griffiths, Morgan etc) suggest this is supposed to be a Welsh mining town. However, Jill Haworth and David McCallum speak with Northern English accents, the policeman speaks with a cockney accent and American actors Robert Doyle and Constance Cavendish speak with unspecific accents.
The entire story line illustrates the misconception that evolution is a directed process leading to improvements in a species. The actual processes of evolution were known long before the time of original airing.