Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Planet of the Apes
The expansion on the lobotomy of Landon in the 'Conspiracy' novel. Even without film to accompany it, it's still freaky.
Pretty much the entire second half of Beneath The Planet of The Apes. Take your pick. From the freaky hallucinations of the upside-down, crucified gorillas and giant statue with bleeding eyes to the two heroes being forced to kill each other in a cell lined with deadly spikes to the mutant humans pulling off their faces to reveal themselves as hideous, skinless abominations (the concept of mutants worshiping the source of radiation is pretty nightmarish in itself) to finally end with the destruction of the entire planet and a completely nihilistic closing narration ("In one of the countless billions of galaxies in the universe, lies a medium-sized star, and one of its satellites, a green and insignificant planet, is now dead"). Any kid who sees this film won't get any sleep that night. And to think that this was a G-rated film.