Just really unpredictable stuff. Like an enemy comes off screen and kills you. Or you have to perform a jump in a side scroller.. and you fall to a lower level.. right into a lava pit or something. No way to possibly know that it's there without playing the game at least once.
In general, stuff that's just incredibly frustrating and seemingly there's nothing you could have done to prevent it.
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http://www.last.fm/user/VinnyMendoza
"This is all we have... when we die.."