This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Don Hertzfeldt
The robot that looks like a smiling mouth.
The hallucination scene from everything will be ok, mainly the parts where the entire screen is covered by flashing lights and shapes, along with what looks like a blurry, deformed pig's head flapping its jaws in the foreground. All the while the music becomes more frantic and jarring until it's basically nothing but static and noise.
Wisdom Teeth is both this and Nausea Fuel, especially as the short reaches the (literal) end of its thread. If you have issues with teeth and dental issues, this is not the cartoon for you.
A lot of the existential stuff in the Bill Trilogy can leave you with an empty feeling inside, especially how almost every time it happens some mundane non-sequitur would occur as a cutting-off point. For example, from "Everything will be OK":
"Bill daydreamed about all the brains in jars he used to see at school, how he used to wonder if there were still pieces of individuals inside... He began to look at people in a new light, how everyone's just little more than that frightened fragile brain stem surrounded by meat and physics, too terrified to recognize the sum of their parts, insulated in the shells of their skulls and lower-middle-class houses. Afraid of change; afraid of decisions; afraid of pain; stuck in traffic, listening to terrible music.
His neighbor stared at him and said, "Last night I dreamed all my toes fell off.""