There is an old dictum in mysticism: Ipsum Nomen Res Ipsa -- "the name itself [is] the thing itself." This is a rule for hypnotizing oneself or others to change our perceptions of the universe to fit our ideas. This rule is the opposite of the rule of science, which is to change our ideas (theories) to fit our perceptions of the universe (observations).
Corollary 1a -- Lincoln's Law: Calling a tail a leg doesn't make it one.
The practical conclusion of the above rule is that we cannot alter reality simply by changing the names by which we refer to things. There are good reasons for changing names sometimes, specifically when we find that the old names do not accurately reflect observations. However, when we change names out of wishful thinking (calling a dog's tail a leg) we set ourselves up for delusion and disappointment.
Worse, when we assent to others' redefinition of the words that describe the world, we are effectively under their spell. Who is doing Black Magick upon you? (What does the word "waffle" make you think of?) Reality is ultimately reality-based, not faith-based, and the credibility gap is a tension between the two. When it snaps, people do get killed.
2. There's always the chance the guy is lying to you.
This insight is famously ascribed to David Hume, but outside of credulous Christendom it may simply never have been needed: Whenever someone tells you that a miracle (or other unlikely event) has occurred, consider the following. There is a probability M that a miracle actually has occurred. There is also a probability L that the person who is telling the tale is lying or simply mistaken. As long as L > M, we have no reason to believe in miracles, wild advertising claims, or other unlikely stories.
3. Popularity and correctness are not strongly correlated.
Corollary 3a: Ten million people could be wrong.
Sometimes ideas are useful, but unpopular -- either because few people have heard of them or been convinced of them yet, or because they have gone out of fashion.
Corollary 3b: They laughed at Gandhi, but they also laughed at Bozo the Clown.
Being original is not, in itself, any guarantee of being right. Likewise, the fact of being rejected is no assurance that you're on the right track. Sometimes, first they ignore you, then they laugh at you, then you figure out you're being a dork and quit it.
4. People who sound totally sure might just be trying to convince themselves.
If a person is absolutely insistent on some point, it may well be that he (or she) is working under the rule of mysticism rather than that of science: rather than trying to come up with statements that accurately describe the world, he is trying to convince himself that the world is how he wants it to be.
It's not always the case, though. Sometimes we find that in order to prevent harm, we need to do some magic or politics -- same thing -- even for ideas that we have discovered by science. Otherwise we end up with creationism in the public schools and pi being declared equal to 3 by legislative fiat. Sometimes we do have to insist that we're right and the other guy is wrong. But we have to offer evidence, not just assertion -- and we have to be careful (not certain, but careful) that we aren't letting our ideas run away with us.