For me, the biggest one was Rare being bought by Microsoft, and subsequently ruining them, for which I'll never forgive them, and indeed that and every Final Fantasy switch up were the biggest ones I can remember experiencing among the gamer community, but from my personal experience, I have NEVER seen as big a meltdown as the Xbone reveal fiasco. I honestly can't think of anything that compares to that. I mean I'm still more pissed about Rare, but for the industry as a whole, that was definitely the biggest meltdown.
And if anything could happen, I'd say the biggest meltdown possible would be another gaming crash, resulting in all three leaving the industry, and no one taking their place, forcing everyone to switch to PC and mobile gaming whether they like it or not.