2. Genital Mutilation
Examples: Hostel 2, Piranha 3D, Grotesque, The Exorcist, Cannibal Holocaust. You'll notice that I've sensibly opted not to include a picture of this one (merely a picture of a man who had it happen to him)... I won't be describing this in too much detail, for obvious reasons, but the mere thought of seeing this happen to someone else (let alone having it happen to you) is enough to make you feel physically sick. As the gore-porn sub-genre of horror grew in popularity, so did the maiming of victims' genitalia, though we can go back as far as the Exorcist (in which is was self-inflicted by the demon-possessed Regan MacNeil) and beyond to see this happening on screen. Early snuff films were also notorious for it - see Cannibal Holocaust for an example - but it is movies like Hostel and Grotesque (movies with nothing better to do than throw blood and guts at you) that are absolutely filled with such scenes. The less said about it, the better, really - but it would quite obviously be incredibly painful to experience.