Keeping a spare could be handy indeed! Once I had a stick get through the spokes of my bike and as the wheel turned the stick, it wedge itself on the frame (oposite the derailer), as one side was stationary the other side (derailer side) rotated and caught the derailer and simply swung it back, up, and around the cassette. Needless to say time I had bent it back, to ride back out of the trail, I only had about three gears it could reach!