Possession of marijuana while in a correctional institution. My case got dragged out and I was in solitary until it was resolved.

EDIT: I'll add to this that marijuana was one of the things that helped me in prison. It really did function as a medicine at times, letting me drift away from the craziness of my situation and think about myself and what I wanted to become and the person I wanted to be. I think that helped me to internally rehabilitate myself and become the person I am today.