Do you remember Z'ha'dum?
That was the climax episode for the third season of Babylon 5, and JMS went to some lengths to keep the episode title secret, not that it ever can be a secret right up to the time the episode is broadcast. There are TV listings published.
So, as the name appeared, all the fans knew that there was a prediction, to the effect that going to Z'ha'dum would lead to death.
JMS could have chosen a different title if he really didn't want to publish that stonking big clue.
The fans on the newsgroups decided that nobody could ever mention an episode name again, lest they spoil the show for other viewers. This wasn't quite crazy: those of us in the UK saw the episode a month before the Americans, but there was a huge dose of "we know better than the author".
I think the River Song character is playing with this real-world history, when she says "spoilers".
I reckon Steven Moffatt is right to take offence. He gave these people a preview, asked them not to talk, and one of the guys blabbed. And, while he's not perfect, I think he plays fair with the audience. There's so much happening that you can miss a clue, and he doesn't explain everything, but I think he has things worked out.
Anyway, you may have heard of the unreliable narrator as a device in fiction. The Pandorica two-parter makes everything that went before an unreliable narration. Everything, all the way back to Totter's Lane. RTD was closing off options like there was no tomorrow, and the way he kept pumping the climax-scale make "Doc" Smith look restrained. Steven Moffett has re-booted the universe, and gives a very personal threat to the Doctor.