Observers missfire, extendet pistons loose their "head" and torches powered by redstone stay active when they are on chunk borders. That happens when the output side of an observer is in contact with a chunk border piston and piston head seperated by a chunk border or redstone cable and block supporting a redstone torch seperated by a chunk border. I dont have screens for the observer or the piston as I wasnt sure how i could report them, but now I know how, I did take a picture of the torch problem.