My wife was making her first night dive in an 80 ft deep artesian well. The group plan was to "go to about 50 ft, swim around, look at stuff." Well, unbeknown to us, something had happened to the well, and the normally clear water was very murky. We had trouble seeing each other, and orientation went to pot. I looked at my wife and saw that she was suddenly developing an uncontrolled descent -- her greatest fear! I reached to try to stop her, but it was too late. I tried to swim down to grab her, but I couldn't go down fast enough. "Oh, well," I thought. "Her instructor is right beside her, so he'll fix it." And as I turned back upright to figure out my next move, my head struck against an overhanging ledge at about 25 ft. What?
Turns out my wife had not gone anywhere. She wasn't going down; I was going up and didn't know it. Lesson: Sometimes it is hard to tell who is moving. Don't assume it is the other guy who has the problem.