Towards the end of a flight with a quadcopter, I started a vertical descent to come in and land. There was no wind on that particular day so I just came straight down.
Shortly after starting to descend, the quadcopter started falling faster. I increased the throttle to compensate, but it only seemed to fall faster.
After a rather unceremonious landing, I checked the quad over and found it still had power enough to take off and hover, so I don't think it outright lost power. It came down level with the motors going.
What would have caused the rapid loss of vertical thrust?