The ending of "Sardines", especially given that one of the characters, Rachel, is claustrophobic. Imagine being claustrophobic and hiding in a wardrobe stuffed full of people, one of is your ex and one of whom stinks, with barely any space. Then you hear someone outside the wardrobe locking its door, and a voice singing a song about sardines, and you can smell petrol fumes...and then the wardrobe bursts into flames, and you're trapped inside, with no means of escape.
"The 12 Days of Christine": The eggs being thrown from nowhere in Christine's flat and the stranger with the raincoat who keeps appearing to her, and seemingly trying to kidnap her son.
"Cold Comfort": George making his way up the hallway on one of the CCTV cameras which is playing alongside the creepy still-image of him being shown on the other camera. All whilst the sound of a disconnected phone plays.
"La Couchette": When the dead man's corpse pops out of his bunk, scaring Shona and Hugo.
"Tom and Gerri": The sudden close-up of Migg's corpse in the bath.
"The Harrowing": Any scene at all that features Andras/Castiel.
The beginning and end of 'Séance Time'. At the beginning, the audience is led to believe that the séance is real, making it genuinely chilling until the reveal that it's part of a hidden camera show. And at the end, it's all played out for real.
"The Understudy": Jim's hallucinations, seeing drips of blood that grow into pools, and blood dripping out of his own eyes.
The twist ending to "The Devil of Christmas".
The entire second half of "The Riddle of the Sphinx". Everything starts to go rapidly downhill after "Nina" is paralysed by the pufferfish toxin.
The Wham Shot in "To Have And To Hold", which reveals that Adrian had been keeping a sex slave in a secret room in his basement for nine years.
Only a few minutes of "Dead Line", the live Halloween episode, could be broadcast, as technical difficulties led to the audio cutting out. Except not. It's very convincing, but the viewer realises there's something wrong when a repeat of "A Quiet Night In" put on in its place seemingly gets corrupted within the first few minutes, and a skeletal figure that wasn't there before starts clawing at the window. After further "technical difficulties" messages, it cuts to the various cameras in the studio. What follows involves macabre clips from old programmes, a supernatural presence, and every variation of Nothing Is Scarier employed to nightmarish effect, culminating in a truly disturbing Kill 'Em All ending. Especially when a skeletal face attacks Reece towards the end.
If you look closely, you can see people hiding in the studio, including a girl in a white dress just before a woman is murdered. Towards the end you see her again very briefly when Reese is in the dark.
The Wham Shot in "Death Be Not Proud" of the rotten corpse of David's mother in the bath. In fact, the episode is made of this trope; it's one of the most disturbing ones yet, from the adult babies to Emily and David bonding over their love of serial killers to Psychoville's Mr Jelly doing a trick with a rabbit that goes horribly wrong. And by the end, Beattie has gone insane, murdered her boyfriend and put his corpse in the bath.