An Angry Ghost
I was visiting my cousin in LA and we were in his room messing around, as usual, when something sharp hit my arm. I looked around to see what hit me and I found a sharp piece of plastic laying on the floor by the arm that got hit. I knew it couldn't have been my cousin, he was on the other side of me.
I thought I was going crazy at first, but then after we dismissed what had happened and had gone back to what we were doing before, I saw a dark figure shift in my peripheral vision. Then as soon as I saw the figure move, a whole row of books on his bookshelf fell off and onto the floor in a messy pile.
We looked at each other and ran out of the room before something else happened. When we got to the living room, the adults asked us what happened because they heard a big crash. We told them what happened and said we thought it was a ghost, but of course they didn't believe us. To this day I still wonder if the ghost was angry at us for something we did or said, or even if it wasn't our fault he was angry at all.
But I'm almost positive the ghost is still there, most likely still bothering my cousin.