A person who is deluded or simply stupid, making mistakes without the intention of ever acknowledging people's corrections. And the common response to these corrections are often outbursts of utter ignorance, hence the play on the word "terrorist," someone who is potentially dangerous in society by simply being ignorant.
Errorist: I don't care what you say, but Tupac will come back in 07/07/07!!!
Errorist's friend: Dude, he's dead, get over it.
--July 8th 2007--
Errorist's friend: Where's Tupac? Idiot.
Errorist: Man you're the idiot. He's waiting for the right moment, you just watch.. he'll come back. Did you even hear about the 7 Day Theory?
Errorist's friend: I don't even know why I'm friends with you. F**king ignorant ass errorist, you're a disgrace to society. I don't want to be your friend anymore..