Empty propane?

I had a propane cylinder run out during my boil. I hooked up the spare tank and continued to boil, but when I picked up the old cylinder it felt like it was still about half full. I swished it around and it definitely has liquid in it. Is this possible? It seems I had the kid who filled one up for me several years ago say something about purging them right the first time they're filled or you have problems. Anybody got any input? I WAS drinking beer that night.

If you got a tank with the new J valves, then it's possible. Those valves are designed to let out propane slowly. If you cranked it up really quick, then it could have shut itself off and you have to wait a few minutes for it to "reset." If you hear a liquid like sound sloshing around in there, then you still have propane. Better yet, weigh the tank and then subtract that weight with the tare weight stamped on the bottle itself.

I had a wierd thing with my regulator on my gas grill. It would sometimes get screwy and would act like it was going out. But I'd reconnect and it was fine. I would imagine your setup has a regulator, so that is something to check as well.