This is from Bangalore, India.
One of the quirks of the city's climate is the occasional isolated spots of heavy rain. There's a running joke of 'When you say its raining in Bangalore, do mention which locality and street too.'
So one summer in the late 90's, my friend and I were in the outskirts of the city, we saw a small crowd blocking the highway. There was a rain front slowly passing across the road, and the people were watching that spectacle. One side of the road was dry while the other side had heavy rain.
So we stopped the bike, cooly got down and took shelter beforehand. And from that comfortable vantage point, we saw a parked goods van whose one side was rainy while the other side was dry. Took 5 seconds for the van to be rained on fully, and another 10 seconds for us to be completely surrounded by rain.
Till date, my memory of this is in a super clear slow motion while my friend doesn't even recall this incident.
Edit: Looks like it is not an exceedingly rare phenomenon, and encountered across various climatic zones. We just have to be at the right place and the right time to sight it.