I'm not easily scared by games, but FFXII had a lot of scary moments as a result of the gameplay mechanics.
There's this insane bonus dungeon filled with ultra-high-level ghosts, and other insanely annoying characters. The first time I went there, I was really underlevelled, and I was surprised by how few enemies there were. Well, as it turns out, they turn invisible, and then arbitrarily attack you when they feel like it. So, one finally comes out, and it is way too powerful. I run like ****. One party member constantly taking off status ailments, one constantly casting Holy, and the one I'm controlling just running for dear life. Then I finally kill one ghost, and the ghost of that ghost comes up to attack me. It's even stronger, and kills my entire party in one hit. While all this is happening, I'm still a mile from the entrance, the other ghosts are materializing and coming after me, and the doubleghost casts a spell which kills off everyone in my party except one. That one has a status ailment called Disease, which prevents healing by lowering your max HP to your current HP. So I start casting Life, which revives someone just before the character dies, then continue the cycle of death all the way out of the level. When I looked back there was something like twenty ghosts just waiting for me to reenter.
And then there's the fact that sometimes you'll run into what is referred to as "rare game", which are monsters that only spawn under certain conditions, and are way more powerful than the other monsters. Sometimes you'll be walking along killing Level 3 dogs when a fifteen-foot dragon runs up out of nowhere. Other times you'll be fighting Level 6 chickens when you find a chicken that doesn't die in one hit. Then you realize he has a different name, and is something like level 50.
FFXII just generally loves freaking you out with ridiculous enemies.