I was facing a wall shelf with a stack of hard plastic cups wedged into it, and when I turned away from the shelf I heard the cups fall to the floor. Based on how tightly they were wedged in there's no way they could've fallen on their own. Although there was an open window that was adjacent to it there's no way that a breeze could've knocked them over, and even if someone on the lower floor punched the wall it wouldn't have knocked the cups down. Nothing else happened in that room that was out of the ordinary.
Although I'm still a skeptic, I don't have a reasonable explanation for this one.