Tombays don't count here - something more unique and different.
While leading a ride on the Platte River Trail in Denver, an entire car wheel from an elevated road next to the trail went whizzing by my head at about 40 mph, bounced on the bank of the river and then splashed into the Platte. Two seconds difference and it would have hit my head. The driver stopped (well, he had only 3 wheels left - he sort of had to) - seems (he claimed) that he had recently had new tires put on, and, evidently, they did not tighten the lugs correctly.
We rode away. I guess the driver had a spare somewhere.