This is a very sensitive subject with me, folks. Although I have been photographing for a long time (perhaps even longer than that youngster, Didger), a few years ago I got myself a nifty Mamiya 6, a 6x6 rangefinder, film camera. It was my first rangefinder camera in many a year, and I found myself frequently encountering those "severely underexposed" shots, usually the first three or four shots on a roll.
Highly embarrassing, to say the least.
In desperation I stuck a couple of Velcro dots on the front of the camera, with a mating dot on a piece of cardboard to block the rangefinder window whenever the lens cap was on. Thereafter, whenever I put the cap on or off, I would also move the rangefinder blocker. Of course, it would have been simpler to glue one of Didger's UV filters onto a hole in the cap . . .