Not once. But twice. My Macbook suddenly turned off while me and my wife were watching some movies. The first time it happened, I thought, well, it is just a software after all and for more than 3 years since I have this laptop, I can count how many times it either turned off by itself or restarted for no apparent reason.
The second time it happened, I thought to myself, this ain’t normal anymore so I treated it as an issue that something was probably wrong with my Macbook. I searched in Google and found out that majority of the people who had the same problem as me pointed to one major culprit. The internal fan.
And when I checked my iStat Pro widget, the fan was at 0rpm. Even my fan widget was not moving at all. The fan went back to normal after I tried to squeeze the sides of the laptop. I think it may have been my palms pressing hard on the areas beside the mouse pad.
Just in case this may happen to your Macbook, you may want to check out the internal fan first.