An entertaining look at some typical logical fallacies and mistakes in reasoning:
Appeal to Ignorance
Such an argument assumes a proposition to be true simply because there is no evidence proving that it is not. Hence, absence of evidence is taken to mean evidence of absence. An example, due to Carl Sagan: “There is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist.” Similarly, when we did not know how the pyramids were built, some concluded that, unless proven otherwise, they must have therefore been built by a supernatural power. The burden-of-proof always lies with the person making a claim.
Moreover, and as several others have put it, one must ask what is more likely and what is less likely based on evidence from past observations. Is it more likely that an object flying through space is a man-made artifact or a natural phenomenon, or is it more likely that it is aliens visiting from another planet? Since we have frequently observed the former and never the latter, it is therefore more reasonable to conclude that UFOs are unlikely to be aliens visiting from outer space.
A specific form of the appeal to ignorance is the argument from personal incredulity, where a person's inability to imagine something leads to a belief that the argument being presented is false. For example: It is impossible to imagine that we actually landed a man on the moon, therefore it never happened. Responses of this sort are sometimes wittingly countered with, That's why you're not a physicist.