Okay, I am likely the most cautious and safe rider around. But, sometimes you can't prepare for everything.
I was riding on the bike path, leading a group of seven seniors on a 29 mile ride.
The bike path is just above the river level, and there is a raised roadway immediately above and to the side of the bike path, supported by square landscaped beams one on top of another, holding the berm.
So, I am happily riding along, and all of a sudden, 50 feet ahead of me, an entire wheel from a passing car on the roadway above comes flying off at about 40 mph, hits the berm and bounces across the bike path and catapults into the river.
One of the riders behind me saw it out of the corner of his eye, but I saw it clearly and closely.
Shortly up the roadway and bike path, we saw a stopped car missing a front wheel. There were people gathered around the car looking at the missing wheel. I and the rider behind me stopped. The driver asked if anyone was hurt, and we said "No."
I asked who had previously mounted the wheel on the car, and he said a shop had done it. Obviously, the lug nuts had come loose from inadequate tightening, whoever mounted the tire.
I am still just a bit shaky. This is one I could not have predicted, and no matter how much defensive bicycling I do, could not have missed if I had been just 50 feet ahead! A wheel at 40 mph would have neatly chopped my head off without a "by-your-leave."