It being October, my thoughts naturally turn to horror and fear. Mind you, I'm a big fan of horror and suspense movies, so it's not unusual for me to be thinking about such things. Of course, I'm also (obviously) a fan of roleplaying games. What I've concluded is that, unfortunately, RPGs aren't a very good -- or, at least, very reliable -- medium through which evoke feelings of horror and fear (or, indeed, any particular emotion). That's not to say that it's impossible to have a creepy or edge-of-your-seat gaming session, but there's no way to ensure this.
In truth, that doesn't bother me at all. I'm increasingly of the opinion that the quest for genre emulation is one of the great banes of our hobby, so the fact that no RPG can guarantee an emotional response of any type isn't a cause of concern. But there have been times when I have played or refereed RPG sessions that did elicit feelings of horror or fear from the participants. Such times are easily replicable; they can't be reduced to a formula, let alone a set of mechanics.
So, for today's question: have you ever had a game session that was memorably frightening or horrific and, if so, was there anything you or your fellow players did that contributed to this feeling? Or was it just one of those things that happened without any real explanation?