Turned up for a lesson, bright sunny day, infinite visibility, no hint of anything wrong with the weather.
Instructor said sorry mate, there's some fog coming, no flying today.
What a load of nonsense, I thought, and went to do the shopping instead. And half an hour later I couldn't see across the road whilst driving to the supermarket.
(There hadn't really been much indication of anything that dramatic being about to happen in the forecast, but the instructor had cheated ... couple of phone calls from mates at upwind airfields.)