Here's an unpleasant story.
One rainy day (one of many; I'm from England) I arrived at the dojo with my large golf umbrella. While getting changed I pointed out that it was around the same size as a bokken (you can see this coming already can't you).
During the practice Sensei was attacking us with bokken for tai sabaki practice. After a few minutes he asked if he could use my umbrella, I agreed.
Thus, we started practicing tai sabaki from an attack by a colourful umbrella. Very surreal.
Unfortunately one of the students didn't get his movement right and caught the point of the umbrella (it wasn't sharp) in his ear, which ripped right through.
All ended well his ear was stitched with no lasting damage. My umbrella was ok once I straightened it. I actually think he was lucky it wasn't a bokken that was used as you all know they don't bend and would probably have done more damage.
A usefull tip for such incidents, DON'T go to the hospital in your gi, they make you wait a very long time. The old serves you right attitude.