It happened Sunday morning. I was at a new place (a condo complex) for a tennis meetup. 3 of us showed up early, and we were hitting around. I decided to use the restroom before the others showed up and play started in earnest. I borrowed the organizer's pool key (he lives there) and located the men's room, unlocked it, and walked inside. The door shut by itself, and there was no light turned on inside. I made the huge mistake of assuming I could use the bathroom with the memory of what I had seen in the brief time the door was open. Sure enough, I was able to do what I wanted to do. Then, I started trying to trace back my steps to locate the door, and got disoriented. It was pitch dark, and panic set in. There was nobody in the pool area, the tennis courts were a little distance away, and there were no windows. I started banging on the walls and shouting. I imagined myself trapped there for hours or days. Maybe the organizer would come looking for me? I started feeling my way around but just reached one dead end after another. Finally, my eyes adjusted, and I was able to see a sliver of light through the side of the door. I got there and opened it. I realized there was a timer-based lighting system, and you should twist the knob as soon as you can see it (with the door still open), or you are trapped in darkness. What an experience!