Not everyone's ready to chuck their old Canon EOS 5D in favor of the shiny new EOS 5D Mark II. So if you're afflicted with one of the rare instances in which the older model's mirror becomes detached, you'll be glad to know that Canon will fix it for free. How do you know if it's detached? It won't display an image in the viewfinder.
Discuss: Has your Canon EOS 5D mirror flipped out?