My first occured near the beginning of the game when I was using flames against some "people" and accidently lit an oil puddle (which I thought was bugged reflection textures in a puddle). Wow was that a cool effect. (Accidently lit myself on fire too!)
Second was when I came into a cave of bandits and two were sitting down. I shot one from stealth and insta-killed him, the other stood facing me with his back to his chair...and I shot him too...and he flew backwards over his chair in a complete cartwheel effect...HAHAHA, twas funny to see.
Lastly (and this is just maybe 10 hrs into the game so far), I decided to try a scroll out on a group of bandits called the "Scroll of Mayhem"...well upon realizing that it takes a few seconds to cast a scroll, I watch the casting animation...WOW...My hands swirled around and then one hand came kinda up then slammed down into the ground...the spell itself shot out in a radiant circle around me, somehow lighting an oil puddle nearby, but, just the effects....I dunno, reminded me a lot of the force/lightsaber stabs into the ground seen in STAR WARS....this by far was my favorite...I had to replay this effect over and over again...indeed it WAS mayhem afterwards...hehehe.
Anyways, how about the rest of you who can play the game? (sorry for those who can't yet, the game is worth it!)