My favorite pop-up was this one time when we were trying to fly from Addison to Tallahassee, and had to divert to Montgomery, Alabama to wait out storms. After sitting for a couple of hours, I saw my opportunity in a well-placed gap between storm systems, and my determination that hood ceilings existed along the relatively short route.
Well, reported weather's one thing, reality's another. As I scooted along, the 2500' ceilings became 2000' and were nudging downward, while the sectional showed me that there be towers ahead. Still smooth air.
I was on FF with Cairns Approach (Army), and asked if I could get a pop-up IFR. He asked "what altitude you want?" I told him "7500," and he said, "November miner zero three victor is cleared to Tallahassee via direct. Climb and maintain seven thousand five hundred." It was a great relief. Nice guy, wish I'd sent him a bottle of scotch.
In the entire time I was in his airspace, no one else talked to him.