About 15 years ago, there was this huge storm system that swept across half of Virginia. I was in my room at my then-apartment when I heard a little bit of rumbling outside, so I went out to the balcony to see what was up, and I saw something to this day I can't explain what it was, and whenever I ask someone who knows weather, they can't explain either.
The clouds were thick and puffy as hell, almost like the sky was blanketed by marshmallows, or cotton. And every second, a couple of these "lighting spots" would light up - almost like a disco dance floor, up above. And that continued for about half an hour. Almost looked like there were a fleet of UFO's in the sky trying to communicate or something.
The next morning, when we watched the news, they say that our entire county was ravaged by extreme hail, wind and lightning - even a few tornadoes were confirmed. Everywhere except our town, with the little lightning spots.
EDIT: Here, to help better illustrate, this is more or less what the clouds looked like:
Now imagine each one of those "puffs" individually lighting up on and off. Pretty much that.
The fact that no one can explain it probably means no one else has ever seen it.
Ever. At worst, it's an extraordinarily rare occurrence. And to me, that's pretty special.