I was heading up to San Francisco recently, in the sort of fast but heavy traffic we have around here, when I saw up ahead an odd white cloud and cars braking and swerving. Turned out to be a slab of wallboard in the process of being powdered by all the cars and trucks that were opting to drive over rather than swerve. Suddenly I'm riding over plaster chunks and dust, and the stuff is swirling in my helmet, eyes, nose, mouth. I can't see, I can't breathe, no where to go except forwards.... That was bad. As soon as I got through it I opened up my visor at about 65mph and was turning my head side to side to try to blow it out. No, I was not paying much attention to my riding at that moment.
Made it OK, makes for a good story, but I'm just stupid lucky some car didn't run me off the road in all the excitement, or that I didn't run myself off the road, or skid out on the stuff on the road. whew.