This story is for instrument pilots only. It was a severe clear day all the way from Miami to Pensacola, Florida. There were five people on board my 210. My direct flight path was going to take me 150 miles over the Gulf of Mexico, crossing the ADIZ border twice. Filing IFR was the easiest way to keep from alerting the National Defense System.
Approaching Tampa the controller told me to make a right dogleg to Lakeland VOR. I asked if that vector was to keep me out of Tampa's approach airspace. When the controller said yes, I asked how high I would have to climb to go direct from my present position to Pensacola.
The controller said I had to be higher than 12,000 feet. I was at 12,000 feet. The next IFR altitude was 14,000 feet. There was not enough oxygen, and I did not want to cancel IFR because of the ADIZ problem. Then it hit me. I could go VFR over the top at 12,500 feet. That would allow me to stay on my IFR flight plan, go direct, and not have to use oxygen. I asked the controller for 'VFR over the top' at 12,500 feet.
There were a few seconds of hesitation, then the controller caught on. He chuckled, "Climb to VFR over the top. If not VFR by 14,000 advise."
The controller was aware that it was severe clear. The moral of the story is sometimes you have to know the obscure rules to the make the system do what you want it to do.