I heard it again on national television last evening…….’if it sounds too good to be true, then it is.’ The announcer even said it like…”If it sounds too good to be true………” and left the last part for the audience to fill in….before he finished it himself, affirming that everyone already knew the full phrase. He said it as he nodded his head….knowingly….which is supposedly to denote some kind of wisdom, I am assuming.
I believe another variation might be “Get real!” or is it something like, “Dumb down!” or “Live small!” or maybe, “Careful! You might get hurt!”
It is usually said in retrospect….after the final results of some issue are in.