Years ago I was rattling around northern Norway with a load of squaddies in the back. It was an insertion exercise and the RNAF were looking for us with their 104s. Dodging between the trees and the sparse farmland I was keeping it at about 30-50ft. Come a small fiord with a sheer cliff on my side.
The other side was reasonably flat with similar landscape to that I had been flying over so I tracked over the head of the fiord to the other side. I kept the radalt at 50ft or so as I started over the water. Half way over the radalt jumped and started indicating 1,000 ft. or so. It had unlocked from the water surface and was now registering the bottom of the fiord.
Fly with the autopilot on 200 ft. radalt over a calm sea: looking for trouble.