If you frequent online message boards or social media, you may have encountered something known as Godwin's Law. The 'Law' officially states that 'As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1.' This aspect of the 'Law' is pretty uncontroversial, but in practice the 'Law' is invoked in some circles any time a comparison to Hitler or Nazi Germany is made -- many of which are legitimate -- with the invoker of the Law claiming that the invoker of Hitler has lost simply by making the comparison. Godwin himself says that this is not the intent of the law, and admits that such a move is often fallacious. Rather his intent in formulating such a 'Law' is to get "folks who glibly compared someone else to Hitler or to Nazis to think a bit harder about the Holocaust." In other words, he wants to discourage lightly making such serious comparisons when they often are not appropriate and to foster an environment where such analogies are made much less regularly.
The problem -- with both over-eager Godwin's-Law-invokers and Godwin's goal itself -- is that the seeming surfeit of Hitler and Nazi analogies in the marketplace of ideas are not purely a product of a general lack of circumspection about the gravity of the Holocaust (though some of that exists), but are very often an appropriate, necessary feature of rational thought. And not only within the narrow band of exceptions explicitly carved out by Godwin for comparing similarly genocidal or totalitarian actions to the Nazis.
The Most Extreme X, for any given category or subject, is always a useful referent for the logician. Any time someone makes categorical or absolute claims about any subject, appealing to The Most Extreme X within that field can often immediately refute their claim. For example, if someone says "It is never morally permissible to lie", the most popular refutation by way of anecdote appeals to the Most Extreme Circumstance in the field of morally permissible lying, and it's almost always the case of hiding Jews in your house with Nazi SS at the door interrogating you. This is a completely legitimate and logical move, and its intent isn't to tar anyone with guilt by association. So the fact that this example is invoked over and over again is a good thing, contrary to Godwin's desire to see the use of such analogies decrease.
And this isn't the only situation where such a move is called for and appropriate. The examples are virtually innumerable. Consider someone making a defense of democracy as an inherent good, without qualification. Once again, the rigorous logician is immediately compelled to point out that Hitler rose to power via democratic institutions of government. And again, this sort of move is nothing like the glib, dismissive sort of analogy where someone compares one person who did something mildly bad to Hitler, who also did something bad. Yet the Godwin-Law-invokers will pipe up with their facile objections just the same.
Godwin's Law isn't an actual law, of course, and it was formulated in an at least partially frivolous and sarcastic way, so a serious response to the phenomenon on my behalf may seem disproportionate or silly. But it really does injure legitimate discourse and gives shelter to faulty, irrational dogmas, if it is taken even semi-seriously, which it often is.