The shutter on my medium format camera (a Bronica ETRS) sticks sometimes. Fortunately, there's a little indicator light in the viewfinder that blinks when the shutter has properly actuated, and when it doesn't blink I just flip the double exposure switch and reshoot the same frame. Unfortunately, when I don't see the blinking light I'm never sure whether it didn't actually blink or if I just missed it. So I end up with a lot of unintentional double (or triple or quadruple) exposures. This one just happened to work out well.