Sharpshooter Fallacy

A way of fiddling statistics where you don't establish the conditions until after you've "proved" them. The prototypical example is of a person shooting a gun at a wall, then painting a target around the bullet-hole, and claiming to have scored a bullseye. Karl Popper summed up this fallacy as applied to science with "A theory that explains everything, explains nothing". Basically, if any possible outcome could be interpreted as supporting the theory then it is useless. Which is the pretty much the same thing as the concept of falsifiability.

The inverted variation is "Moving the Goalposts": you can always add extra criteria until the new conditions are not met anymore.

Examples of Sharpshooter Fallacy include:

The so-called "Bible Codes" use this fallacy. Rather than saying what they expect to find in a particular book beforehand, the people who produce these simply manipulate the letters until they find something that they can use. Words count regardless of whether they run up, down, right-to-left, left-to-right, diagonally, or even have the letters adjacent at all.
- This particular case was specifically shot down by a skeptic in a History Channel documentary about such Bible Codes. To prove that such a "spectacularly rare occurrence" actually was more likely than people were willing to admit, he applied the principles for finding codes to Moby Dick, looking for "predictions" of the assassination of JFK. He found quite a few. As with the metaphor of monkeys with typewriters, any sufficiently long stream of data, if looked over using enough different formulae, will produce words or phrases that correlate to some kind of event that occurred after that book was written.
- John Safran vs God put this exact argument against the Bible Code to the test by feeding the entirety of Vanilla Ice's back catalogue (song lyrics and liner notes) into the decoder; even "Ice Ice Baby" is able to turn up 9/11 "predictions." Then they took the 9/11 Comission's report and used the code to find references to the fall of Vanilla Ice's career.
Shakespeare's works get this treatment too. Some who dispute the authorship of his plays claim messages hidden in them reveal the truth when the right cypher is applied.
Similarly, interpretations of the metaphorical elements of Nostradamus' prophecies may be seen as examples of this fallacy.
Likewise, all claims of various people's names being secret encodings of the Number of the Beast, 666. There are a lot of ways you can assign numbers to letters or words—try enough of them, and you will find one that adds up to "666".
- This one is actually referenced in War and Peace, where Pierre plays around with Napoleon's name and titles to make it all add up to the Number of the Beast, then does the same thing to his own name to "prove" that he's destined to assassinate the man.
- This concept is parodied in the Nostalgia Critic's review of End of Days.
Beautifully illustrated in the Principia Discordia with the Discordian Law of Fives: "All things happen in fives, or are divisible by or are multiples of five, or are somehow directly or indirectly appropriate to five."
- Lampshaded in the same section.

"I find the Law of Fives to be more and more manifest the harder I look."

- The Illuminatus Trilogy, being heavily influenced by Discordianism, further deconstructs this. After playing up the significance of the Law of Fives throughout the book, Hagbard Celine then proceeds to demolish it by explaining it as an example of this trope, reinforced by intellectual pareidolia. He goes even further, explaining how even the number five is merely an accident of nature: "If humans were born with six fingers instead of five, we'd be talking about a 'Law of Sixes'".
- Noting that 2+3=5, references to "23" count for the Law of Fives.
  - Basically, the film The Number 23 runs on this. Once you start looking for 23 (or any other number) in creative enough ways, you'll see it everywhere.
    - This is spoofed by The Number 24.