"If someone says the moon is purple, just laugh in their face and move on!"
That is, when you're in a conversation with someone proclaiming things that are incorrect, don't waste time validating their "side" by arguing about fact as though it were a matter of opinion. I think I'll adopt the purple moon phrase as a useful shorthand for this.
(In the dream, I was giving this advice to Hillary Clinton, Mike Wallace, Ed Bradley and Bill Moyers. It was fun!)