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Nightmare Fuel: Inspector Spacetime
Inspector Spacetime's been on the air for over 50 years. That is also the amount of time the programme's monsters will keep you awake at night. Not for nothing is it said that kids can watch only from behind the sofa, with their hands over their eyes, while wearing a welding mask that blocks out 90% of all light.
The Blorgons. They may look stupid, but they will give you nightmares.
Clothons. Just think about it. Waking up one night as you find your own pajamas strangling you to death, being attacked by underwear in a retail store, forced to wear clashing colours....
The Quiet Men and the fact that they won't shut up, especially concerning the Ironic Nursery Rhyme about the Inspector's impending doom.
Ring ring, goes the bell/
One day we all must quit/
Ring ring, goes the bell/
Till the Inspector snuffs it
As soon as you see a Quiet Man, you instantly are stricken mute, leaving you wide-eyed and helpless as it gets ready to blow you to pieces. Oh Crap.
The Cacophony. They engulf your mind with disturbing sounds of their feeding on you, your friends and family. The more there are, the less you can think about anything else, until your mind simply overloads and shuts down, giving them the opportunity to feed on your comatose body.
The Sandmen, from "The Deserts of Venus". Humans who have been infected by a silicate-virus, turning them into shuffling zombies with sand and dirt constantly pouring off them. The sand gets into machinery and clogs up the insides, rendering them useless. Most terrifying, however, is their method of infecting others: They bury people in the sand they constantly produce, effectively mummifying them, and then the infectee bursts out of the pile of sand in typical zombie-esque fashion.
The Hollow Men, from the Ninth Inspector two-parter "The Cambiare Machine"/"A Spacetime Musical". Their Black Eyes of Evil are literal voids. They're walking bombs whose shrapnel turns people into more Hollow Men. And their screeches are... enormously creepy.
The Inspector-Muppet in "Mindscrew" was rather adorable. The Final Stage puppets... not so much.
Chilopodophobes argue that although the Creepy Centipedes from "The Monster in the Crevasse" were intended just to provide a little atmosphere, they're scarier than anything the show has done before or since.
The Snarling Lions. Does this even need explaining? Many a fan has found themselves unable to watch The Lion King ever again.
The Sergeant verbally dissecting Joanna's family and the de-aged Inspector as he holds them hostage during the dinner party. Julian Sands' subdued performance as the Sergeant lends some legitimate creepiness to the performance as he, one by one, brings all of the Martin family to tears without even leaving his seat. Even worse: it's implied that he did this to them every night during The Month That Was Meant To Be, while holding them hostage on his submarine.
The Humiliation Conga at the end of "Anger of the Inspector" when the Inspector forces the Haemo Clan to undergo horrendous punishments. Haemo Daddy having to spend the rest of eternity in the form of a Mexican luchador's underpants was bad enough, but the punishments of the rest of the Haemo Clan are too horrible to mention.
The climax of part one of the Snarling Lions two-parter Stare Into Darkness/See No Evil, where the lights come on and the Inspector realizes that the darkened building that he has just chased the Lion into is a house of mirrors.
Yeah, yeah, they did "The Gloom of Aquanus" when they were going over budget on sets, but the sequence with that Shadow Vector thing is still damn creepy.
Especially when it shows up in the White Void Room at the cliffhanger before the final episode.