This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Sapphire and Steel
The basic concept of the show: the universe is constantly at risk from hostile temporal attacks, and the best any human involved can hope for is that the "nice" Humanoid Abominations don't decide that killing you is the best response.
How so many of the menaces on this show are ones that ordinary human beings can't even recognize. never mind fight back against. And even the superpowered Humanoid Abomination experts who are assigned to stop those menaces have to use an alarming amount of trial-and-error - often costly and painful error - to get a handle on what they're up against.
The prison and eventual fate of Ruth and the landlord.
Even though Ruth's friend Liz survives, she will have to take precautions for the rest of her life, and if she slips up even once the monster will be able to get at her if it ever escapes from its can — even if it doesn't escape until long after she's dead.
Harsher in Hindsight - Good luck to her following Steel's advice with the increased prevalence of CCTV and ubiquitous consumer photography e.g. smartphones over the past 30 or so years.
Although that may actually be helpful for her, as the Shape's affinity for paper suggests that it's only actual photographs - i.e. pictures created on chemically-permeated paper or film - that he inhabits and can manipulate. So it's possible that a digital image would lie outside his "habitat". Digital photography was already being used by NASA when the episode is set, yet Sapphire didn't pick up any impressions related to space probes or radio transmissions from the Shape, only old-tech darkroom sensations.
Just to make it worse, consider the agents' discussion of how the Shape could be lurking behind things in photos, like on the other sides of walls or behind bushes. Presumably that means that, to the Shape, a photo of a house would contain all the interior features of that house - rooms, furnishings, etc - that aren't visible to a person looking at the picture of its exterior. Now, consider how such "interior features" might include all the people in the house. From the Shape's perspective, poor Liz could be accessible via the concealed back rooms of photographs taken from the street outside - or even from a plane flying overhead! - that she never even knew existed...
The photo-kids may have been nasty little brats, but one can't help but feel for them when they're dragged back into the old photographs the Shape released them from. Particularly if you assume they'll remain conscious in there, same as Ruth was sort of aware of herself when she was trapped, or that the Shape may have called up and enslaved the souls of the long-dead people who'd posed for those photos, same as he'd threatened to do to Liz.