Have You Met Fallacy Man? Here’s How to Defeat Him.

I've met the mistaken form of identifying an "ad hominem" fallacy here on HN more times than I can remember. Usually, when one participant here quotes an "expert" to prove a fact about the world, and then someone else calls into question the "expert's" basis of knowledge (a very legitimate thing to do in a court case, for example), a third participant will say "That's an ad hominem argument." And often that last statement is simply wrong. Blind people with their eyes closed in the dark are not reliable witnesses to the color of the suspect's clothes. Deaf people with their ears stopped up next to a waterfall are not reliable witnesses to what someone said in a whisper. People who are idiots about science are generally not reliable about subtle facts of nature. And so on.