This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Beavis and Butt-Head Do America
The infamous hallucination trip scene is a deranged display of Nightmare Fuel. This includes, but isn't limited to, Butt-Head's skin melting off, demonic versions of the duo eating and deep-frying little monsters, and an undead version of the two literally headbanging their skin off.
The video footage of the X-5 unit's test subjects. In order, we are shown: One subject vomiting repeatedly on the floor, the next writhing in pain, and the last subject already dead and having the covers pulled over his face by a person in a hazmat suit. One of the agents watching the video then run off to vomit.
Gigantic Butt-Head becomes a bit of Mood Whiplash though, as the first words out of his mouth in this sequence are a chuckling "This is cool!" And what does he say to the screaming, terrified woman he grabs? "Uh... hey babeh... I'm like... pretty tall!"
Muddy Grimes is pretty damn creepy when he's enraged.
Hell, he even manages to briefly scare both Beavis and Butthead into silence when he shoots the TV, considering how the two teens normally are, that's something.
Beavis's psychotic rampage after taking mystery pills.
The fact that in the desert, Beavis and Butt-Head were on the brink of death from dehydration, collapsed and accepting their imminent death, being hungrily eyed by several vultures. Immediately becomes a funny image when Beavis smacks one of them away, saying "Cut it out, butthole!"