This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / How It Should Have Ended
The The Wizard of Oz HISHE is a mixture of this and a Tear Jerker: Dorothy returning to Kansas before she can journey down the yellow brick road leaves grim fates to the story's other heroes. The Scarecrow remains tied to a pole in the cornfield, and gets pecked by crows who he fails to scare. The Tin Man becomes rusted so stiff, he can not move or talk. The Cowardly Lion never finds self-confidence, and quivers at every sound the forest makes. The Wizard then creates Mood Whiplash as he tries to cheer up the viewers.
Eh, don't be sad. I didn't really ever give them what they asked for in the first place!
Angry Gwen in the one for The Amazing Spider-Man. When she turns red and yells "WHAT???"
The part in the Batman Begins episode where Rachel's face completely changes due to Warner recasting her for the sequel comes out of nowhere, and even scares Bruce Wayne. The dramatic music accompanying his reaction doesn't help.
And then there's The Stinger, where Gordon, when he's told that the compound in the water causes hallucinations when ingested, mentions that's why his wife flipped out on spaghetti night. Cue his wife lying on the kitchen floor screaming as Joffrey appears in the window sporting a creepy Slasher Smile as the walls turn black for no effing reason as ominous music plays.
Whoever was voicing Ultron sounded way too close to James Spader. Like in the trailer, the way he demonizes famous lines from Disney is equally as chilling, no matter what sort of comedy is being shown. The Avengers show nothing but fear. (Though Hawkeye's reaction to the quote from Frozen is actually kinda funny.) There's also the scare chord near the end.
(With no music at all and completely black) Now, I'm free...
Ultron putting his arms around DCCU!Batman and DCCU!Superman while creepily quoting Toy Story. Even Superman is freaked out.
Batman's casual reaction just made it even creepier.
Spiderman's freak out at the end of the The Amazing Spiderman 2 trailer, going so far as to threaten to shoot Batman in order to get Superman to turn back time, topped off with a helping of Mood Whiplash when Superman actually does it.