I'd say the first time they did the meta thing was when the Winchesters found out about Chuck and his Supernatural novels. It was kind of funny then because they didn't grind it into the ground and the idea of a pulp fiction writer actually being a prophet was pretty clever. Then they fucked it all to hell with The Real Ghostbusters, which was a nasty, offensive pile of shit, and they even destroyed the modest cleverness of Chuck by making him...God or whatever he was supposed to be in "Swan Song."