Arx Fatalis/YMMV

• Demonic Spiders: Ylside: they are extremely fast and do massive damage. Their only real weakness is magic, and if you haven't invested enough in that (or have no equipment to make up the deficient) fighting them can become borderline Unwinnable unless you exploit Good Bad Bugs.
• Game Breaker: The Bless spell boosts caster's all stats (on x+1, where x is his level, and since skills depend on stats, they, too, increase) roughly for twenty seconds. Certainly enough time to pull on some heavy armor or powerful weapon (and it won't fall off when the spell effect goes out), see some well-hidden door, pick some complicated lock or cast some top level spell.
• Goddamned Rats. These pesky rodents are everywhere(seriously, there are only a couple of locations without them), make extremely annoying squeaks when they notice you, are small and able to dodge your attacks, and so are hard to hit with any non-BFS weapon. Thankfully, they only deal Scratch Damage. Why the hell did not Black Beast eat you there in dwarven mines, you little bastards?
• Good Bad Bugs: Some of the players experience a rather unusual bug where the protagonist's movement speed appears to depend on the framerate, which is not capped. On top of that, speed is the only thing affected in this manner. This makes fighting in narrow tunnels (and navigating them, which is one of the larger parts of the gameplay) less tedious than it generally is, although players with high-end rigs complain that sometimes they come close to breaking the sound barrier.