Normally, I would have posted the following video in the Leica diary section, but that section is about totally serious Leica subjects only ;-). What the guy in this video is demonstrating (I hope) to the videographer, is that it's easy to forget that you put the lens cap back on the lens. With (d)SLR's you don't have this problem, since you're looking through the actual lens (unless you're blind or shooting in pitch black).
Fairness to say that I've had it happen to me a couple of times, but I notice my mistake at the moment I press the shutterbutton (+3sec exposure in broad daylight?? Must have forgotten something...).