The three times it happened:
1st - switching from normal mode to power saver mode, the "switching to power saver mode" dialogue box popped up and never went away. I navigated to other screens and the apps and it never went away then just shutdown.
2nd - Watching an embedded youtube video in another forum.
3rd - Playing Shadowgun with my BT controller. I was not doing anything in the game, I actually walked away to look at somthing on the TV and when I came back it had shutoff.