Heh. On my second playthrough, my mage wasn't careful about whom his spells hit. I was fighting some brigands, on nightmare, at level 11, and a rabbit attracted one of my magic missiles. BANG! Some level 22 killer bunny shows up!
It took me at least 5 minutes of kiting and ranged combat to kill it. And now I wonder - is the bunny always level 22, or is it twice the player level? I.e. did I just miss out on A LOT of potential experience by killing at level 11 something I that woudl have grown to be a level 70 monster?