Which law says online parody can be mistaken for sincerity without clear cues? The answer is Poe's law.
Poe's law describes a recurring problem in online communication: without an unmistakable signal of intent, a parody of an extreme belief may be read as a sincere statement. The reverse can also happen, with a genuinely extreme claim dismissed as satire. Text-based discussion makes the problem especially likely because readers cannot hear tone of voice or see facial expression.
The principle originated in a 2005 discussion about creationism on ChristianForums.com. It was later generalized beyond religious debate to politics, conspiracy theories, trolling, and social-media humor. Emoticons, labels such as “satire,” or an explicit explanation can provide the cues that Poe's law says are otherwise missing.
Poe's law is not the same as Godwin's law. Godwin's law concerns the growing likelihood of Nazi or Hitler comparisons in arguments, while Poe's law concerns uncertainty about whether an expression is sincere or ironic. The name honors Nathan Poe, who formulated the modern version during that 2005 forum exchange.