A: Cycling: racing as fast as possible down an untested hill only to find out you can't see when the vibrations make everything jump around in your field of vision.
B: Driving: driving in rain so thick I could not see where I was going. I think the only thing that stopped me from running into anyone else (since I was never sure where the lane lines were)
was the strong probability that everyone else had pulled off the road.
C: BikeForums: posting this.