Well I took the Z70 and the mirror cog didn't fail... but the electronics did. I took about six shots on one day and the next time I took it out to use the shot count had gone up to about a dozen. I turned it on and it soon became apparent that pressing just about any button on it fired off a shot - adjust aperture: shot, change exposure mode: shot, etc etc. I messed arounda little but when it got to about 22 I gave up and removed the roll to avoid wasting one of my last rolls of the original Acros.
I haven't gone back to it. I suppose it could be a low battery but I'm hesitant to risk wasting anything else in it now, and I need a film-end retriever to be able to use the rest of that roll in another camera.