"Extraordinary claims require extraordinary evidence." - Carl Sagan (Wikiquote, which is generally reliable in evaluating whether a quote was really made, and a link to a page that has a video clip of Sagan saying it)
Occasionally I come across answers that make extraordinary claims without any references to back them up. Rather than providing a flag with a complaint that the answer is wrong, I merely state that they are unreferenced. Is that appropriate behaviour?
Related question: Should we have a "back it up" rule for answers?