The first one I had that didn't scare me was the Subliminal messages thing.
The first one to scare me was the Jingle Bells Reversed...
I was in my computer room, didn't know what to expect, then a face screamed at me. I screamed (apparently) and my parents were wondering what happened. My sister and mother thought I hurt myself. My dad, urinating off the porch thought, and I quote, "Someone Chopped their testicles off".
This happened when I was 15 or so. I can't get myself to follow videos with creepiness factors anymore. I have to check them, either by scanning and ruining the surprise, or standing 30 feet away.
and sometimes, that's not enough.