With the current 536 preview, I once lost the People app. The app list said it was already pinned to the start screen, but it couldn't be found. Apparently I had fudged the live tile once when trying to open it, and that was when it disappeared. As I recall, after a reboot it appeared on the bottom of the screen, or else it was always there and I hadn't noticed it before the reboot. At the time, I thought I had inadvertently sent it into another live tile, and created a group folder.