This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Monty Python's Flying Circus
Even the studio audience was shocked seeing this scene...
A few of Terry Gilliam's animations. Those expressionless faces of the people on the photographs can be quite terrifying.
The one where various cut out photographs are eating people is pretty horrifying to watch.
Most of the sketches feature insane people or completely crazy situations you really wouldn't want to go through with in Real Life!
The "Buying A Bed" sketch: imagine desperately trying to buy a bed and having to deal with these impossible to cope with salesmen!
"The Cheese Shop" sketch: Again: trying to buy cheese from a man who doesn't sell cheese at all and is just tormenting you on purpose!
Also the fact that the customer resorts to cold-blooded murder.
To be fair, the shop owner was being a prick for saying that his shop had cheese, despite that being a total lie; one may think that as a Karmic Death.
"The Barbershop Sketch": Apparently the barber murdered all the previous clients for the sole purpose that he is desperately afraid of cutting hair!
Also it's revealed that he wanted to be a lumberjack and a woman. Sounds like someone you know? Goodbye horses, I'm flying over you...
The "Déjà Vu" sketch is probably what insanity is actually like.
The final season sketch involving Count Ferdinand von Zeppelin features a brief bit at the beginning where his brother, Barry (Terry Jones), "the least talented of the fourteen Zeppelin brothers", tries unsuccessfully to blow up balloons. The final one, a huge balloon, ends up inflating him to the point where he floats away.note read: it was a specially-made balloon This in itself is more humorous than scary, but the quick cuts used to simulate Barry being blown up are very good at making it seem like Jones is actually being blown up... in a rather startling manner...
"The Undertaker Sketch", where John Cleese is suggested to eat the corpse of his own mother. And he agrees!
The sketch where a hospital forces their patients to work for them, despite being so fractured that they are all put in bandages and barely able to walk without crutches.
There is a sketch in the fourth season where a man visits the doctor and is stabbed by a nurse. Despite bleeding all over the place the doctor forces him to fill in a form first.
"The North Minehead Election": the idea that Adolf Hitler... sorry Mr. Hilter... is still alive and planning to be re-elected to start another war is rather disturbing.
Although it was a pretty obvious jab at all those who believed Hitler was actually still alive, because they Never Found the Body.
And "Hilter" was obviously getting nowhere in his plans.
The Visitors, which features Graham Chapman's character's quite evening in with his girlfriend getting repeatedly gate-crashed by a group of annoying and even downright horrible people. For most of the sketch, he stands helplessly at the side, watching these people invade his living room, and when he finally stands up to them all, he gets shot dead.
Also counting as a Tear Jerker: The Timmy Williams Interview sketch. Imagine going to your friend in your hour of need, only for him to dismiss you and pay no attention to you, even when you start sobbing in front of him. And when poor Nigel finally breaks and shoots himself in the head, Timmy shows no compassion at all, and is more concerned about whether they got the shot on camera, and his own image.
The Funniest Joke in the World is a sketch about a joke that is so dangerous that reading or hearing just one word of it is enough to send you to a hospital, and if you see the entire joke you will die laughing almost immediately. It even killed its creator after he first wrote it down.