I remember watching a program on TV when I was a kid about a physically-violent poltergeist that terrorised a young boy.
The thing that bothered me the most about what I saw wasn't the fact that the poltergeist could move things and hurt him, but the fact that he couldn't escape from it. His family would move across the country to different locations all throughout his childhood, but the poltergeist followed them everywhere.
The strangest and creepiest part was, that the poltergeist would stay dormant for a few months after they moved to a new place, and then it would announce its presence on a random night with a particularly loud and violent outburst (throwing chairs, turning lights on and off, making the electronics hum with strange noises, shaking the mattresses, etc).
That concept still terrifies me now, and even more so when I was a kid. The idea that a horrible thing could stay with you for that long and terrorise you when you least expected it. Had nightmares for months afterwards, and I still think about that TV program to this day. I kinda wish I knew what channel it was or what show, but it was such a long time ago and I was really young when I watched it.