Fallacies

Ed Feser points out, as I have in the past, that an appeal to authority generally is only a fallacy when the authority isn't really an authority at all. (There is the minor case when someone mistakes it for a deductive proof, but that essentially never happens.)

Furthermore, he notes:

Similarly, not every ad hominem attack -- an attack “against the man” or person -- involves a fallacious ad hominem. "Attacking the man" can be entirely legitimate and sometimes even called for, even in an argumentative context, when it is precisely the man himself who is the problem.

Incorrectly accusing an opponent of one or both of these fallacies is a favorite last resort of the Internet jerk. He shows up in some comment section, and claims, "That's nonsense: everyone knows electrons move at the speed of light!" When someone finally gives up arguing with the fellow, and notes, for the benefit of onlookers, "Well, every physicist in the world disagrees with you," he is sure to be met with the invective: "Appeal to authority!"

But this is a good appeal to authority: physicists know what they are talking about on this topic, and the Internet jerk doesn't.

The other accusation arises when someone evades all attempts at counter-argument. The person trying to reason with him finally declares, "You are being an idiot!" Immediately, he will hear: "Ad hominem!"

But, as Feser says:

There is in such a case nothing wrong with calling such a person an ignoramus, a crank, a troll, etc. and refusing to engage with him any further. That is certainly an attack on the person, but it is no fallacy. It is just a straightforward inference from the facts, a well-founded judgment about him and his behavior, rather than a fallacious response to some argument he has given.