Obey The Walrus
A messed-up YouTube video with a creepy tap-dancing walrus that looks like it has polio and is dancing to some weird backwards nursery rhyme. It'll haunt you so bad you'll scream in your sleep.
I watched it once and now I see a walrus in my shower.
My sister had to sleep with the lights on for a week after that.
I tried to watch it during a thunderstorm and it was like hell on earth.