The phrase "Extraordinary claims require extraordinary proof" springs to mind.
Actually, that's a phrase I really hate, even as a skeptic- proof is proof. Yes or no, it's proved or it isn't. The "extraodinary proof" phrase really sounds like you have more than prove something, and that immediately gives the flimflam artists an excuse to say "well we could prove it but the other side always changes the goalposts"...
Any claim requires proof, end of. Not extraordinary proof because there is no ordinary or extraordinary proof - you've proved it or you haven't.