In my class tonight, we viewed several pieces of Francisco Goya's artwork, and I noticed an effect it had on me that I've never experienced with any sort of classical artwork, or static artwork of any kind, really.
It disturbed me, on a very profound level. It literally made me physically uneasy, and a little frightened.
Especially this piece,
Saturno devorando a un hijo (Saturn Devouring His Son)

Everything about it makes me shudder. The detail of the half-eaten body, clearly drawn from the scenes of violence that Goya saw every day in his war-torn country during the era of Napoleon, the howling madness of Saturn's eyes, filled with not even any sort of conscious maliciousness, but washed over with a complete lack of regard for, and even incomprehension of, the horrific act he's perpetrating.
tl;dr: Good God, this painting absolutely frightens me.