I'm reminded of an experience in college. I had a glass-front stereo cabinet. One day, while I happened to be looking at the cabinet, the glass door shattered. There was no impact, it just shattered. And, to make it more exciting, all the little pieces shattered afterwards. In fact, for nearly an hour, I watched while pieces spontaneously shattered into smaller pieces. Kinda like watching corn pop, only not.
I still don't really have any clue why that happened, nor have I encountered anyone who has had a similar experience. It was very very bizarre.
UPDATE: Almost 300 comments so far, mostly from people who have experienced this same phenomenon. See especially THIS COMMENT which contains a fairly good explanation of why this happened. Please read it before posting YET ANOTHER comment about how nobody knows why this happens. Plenty of people know.