Back when I lived in Boston there was one apartment that had a persistent leak in the bathroom ceiling. At first I would just notice water droplets on the sink and splashed onto the mirror. It was kind of a mystery as to where they were coming from. Then one night I went in and turned on the ceiling light and noticed it was dimmer than usual. I looked up to see that the frosted glass globe was half full of water. It turns out the woman in the apartment above me got in the habit of showering with the curtain on the outside of the bathtub. My whole bathroom ceiling was waterlogged and had to be replaced.