There are two ways of stating Poe’s Law, the weaker one being:
“There always exists a person who will mistake a parody of extremism for true extremism.”
To state the converse, that someone will mistake true extremism for a parody of itself, is Poe’s Corollary.
The stronger form of the Law is:
“It is impossible for anyone to be certain that a parody of extremism is in fact a parody, if it is not explicitly marked as such.”
I would extend that to say that other side channel information (e.g. being familiar with the author of the parody) can also be sufficient to confirm.
A dubious parody of an extreme view is sometimes called a “Poe,” following from this law.
This law was coined by Nathan Poe on christianforums.com in 2005 after a lengthy debate about creationism vs evolution, and originally was only applied to Creationism and parodies thereof. However, citations of the same principle go as far back as 1983 (Recall Stigler’s Law from yesterday).
I’m unable to provide any information about Nathan Poe, since it’s a common name shared by several notable people and his activity on christianforums.com, though extensive, reveals little; but this Poe may be an author of several books on Amazon that have never been reviewed by anyone.