Poe's law: if you don't tell people that you're joking, someone will take you seriously.
This law was originally coined by Nathan Poe, who was commenting on a thread on christianforums.com discussing creationism. In this post, he stated: "Without a winking smiley or other blatant display of humor, it is utterly impossible to parody a Creationist in such a way that someone won't mistake for the genuine article."
This is by no means a new idea. While Poe made his observation in 2005, Usenet user Jerry Schwarz had an post on the same subject back in 1983:
"Avoid sarcasm and facetious remarks. Without the voice inflection and body language of personal communication these are easily misinterpreted. A sideways smile, :-), has become widely accepted on the net as an indication that 'I'm only kidding'. If you submit a satiric item without this symbol, no matter how obvious the satire is to you, do not be surprised if people take it seriously."
However, Poe's law is now firmly established as the common term for this phenomenon, and it has spawned some related terms: a Poe is a person, post, or page that could cause Poe's law to be indicated. Contrariwise, Poe's Corollary is the idea that "any new idea or strong statement will be suspected by someone of being a joke".