I would have to say Higurashi no Naku Koro ni. This was a while back, but let me explain.
I jumped right into the anime when I had it on demand and did not bother to read about it beforehand. At first it looked like an innocent anime; kid moves into town, life in a new city goes to school, the works :) safe and simple. Boy was I surprised when.... (Next is a Spoiler)
everything goes to hell
complete insanity ensues; in this anime....
it sucks to be anyone, cuz you will get violently murdered and taunted and tortured by people you know; I wasn't expecting that when I decided to watch this anime the first time @InVinsybll