Surprised nobody mentioned this one yet. What about at Mania (was it X7? It was in the Houston Astrodome, I know that much) when Austin turned heel by siding WITH Vince, and he went absolutely batshit with a chair on The Rock?
How about HHH turning babyface at 'Mania 22 simply because the crowd hated Cena so much?
Also, the heel turn by Jake The Snake on the Ultimate Warrior, siding with 'Taker. Then, Undertaker's subsequent babyface turn on Jake a little while later.