You could trace this back to the Baron Samedi incident as noted above, or to the '50's s-f flicks that ended with ? or The Beginning. I think, though it's been a long time, that the schmuck at the end of NIGHT OF TERROR (1933) who speaks directly to the audience, rises from the dead to make his comments. But, for all real intents and purposes, the '70's-'80's-'90's trend springs directly from CARRIE. That ending was so powerful and so influential that virtually every English language horror film released between '78 and, say, the early '90's, imitated or riffed on it. And I mean that literally--virtually every horror movie cribbed that final jolt from De Palma. I remember seeing some slasher horror in the '80's that didn't follow the CARRIE rules and I almost suffered whiplash in the theatre from the non-event. And, no, I couldn't possibly remember what that movie was.