I described this exact same thing happening with an aquantences M9 and posted this in another forum quite a fw months ago. Although random in occurance, I could often provoke it (not intential) in his camera by firing quite a few consecutive single shots fired one after the other while the camera was in single shot mode (firing multiple shots in this mode of same subject). We elminated the possibility of low battery, not pressing the shutter firmly anmd a host of other possible variables. It sounds like once the shutter button is pressed, you hear a soft click but no whirl of the recocking mechanism...almost like if one was in decrte mode but when you let go of the shutter button after firing, nothing else is heard (except camera is not in decrete mode). A black (blank) image comes up on his LCD screen. It's like something in the shutter mechanism or some sort of mechanical linkage is slipping ...again akin to firing the camera in descrete mode but after firing, no recoking of shutter sound (the whirl) is heard).
This may be a long shot, but I wonder if if like the cracked sensor M9 issue which seems to be clustered around early serial #'s...as to whether those that experieced the above, also have earlier serial # cameras and Leica addresses these issues after a time when receiving mutliple reports of cameras having an issue. Just a thought. The aquantence who's camera it is, hasn't sent it in as of this date.
Dave (D&A)