A simple cautionary tale comes to us from the UK, with the story of Kim Yeates. She's a Somerset woman who had a 4" crystal ball sitting in her windowsill. What happens when sunlight gets refracted through glass? Well, in Yeates' case it caused £10,000 worth of damage to her flat.
Yeates is now in temporary accomodcations, but because she had no insurance, she's lost pretty much everything. No one was injured, thankfully, but a neighbor climbed in to try to rescue Yeates, who wasn't home at the time of the fire.
While this is probably a pretty rare occurence, if you've got a big chunk of glass sitting in a sunny window, you might want to reconsider the location.