The 23 enigma refers to the belief that most incidents and events are directly connected to the number 23.
I first heard of the 23 enigma from William S Burroughs, author of Naked Lunch, Nova Express, etc. According to Burroughs, he had known a certain Captain Clark, around 1960 in Tangier, who once bragged that he had been sailing 23 years without an accident. That very day, Clark’s ship had an accident that killed him and everybody else aboard. Furthermore, while Burroughs was thinking about this crude example of the irony of the gods that evening, a bulletin on the radio announced the crash of an airliner in Florida, USA. The pilot was another captain Clark and the flight was Flight 23.
Burroughs wrote a short story in 1967 titled "23 Skidoo." The term "23 skidoo" was popularized in the early 1920s and means "it's time to get [leave] while the getting is good." It appeared in newspapers as early as 1906.
The Principia Discordia states that "All things happen in fives, or are divisible by or are multiples of five, or are somehow directly or indirectly appropriate to 5"—this is referred to as the Law of Fives. The 23 Enigma is regarded as a corollary of this law, since 2+3=5. It can be seen in Robert Anton Wilson and Robert Shea's The Illuminatus! Trilogy (therein called the "23/17 phenomenon"), Wilson's Cosmic Trigger I: The Final Secret of the Illuminati (therein called "The Law of fives" and "The 23 Enigma"), Arthur Koestler's Challenge of Chance, as well as the Principia Discordia. In these works, 23 is considered lucky, unlucky, sinister, strange, or sacred to the goddess Eris or to the unholy gods of the Cthulhu Mythos.
As with most numerological claims, the enigma can be viewed as an example of apophenia, selection bias, and confirmation bias. In interviews, Wilson acknowledged the self-fulfilling nature of the enigma, implying that the real value of the Laws of Fives and Twenty-threes lies in their demonstration of the mind's power to perceive "truth" in nearly anything.
When you start looking for something you tend to find it. This wouldn't be like Simon Newcomb, the great astronomer, who wrote a mathematical proof that heavier than air flight was impossible and published it a day before the Wright brothers took off. I'm talking about people who found a pattern in nature and wrote several scientific articles and got it accepted by a large part of the scientific community before it was generally agreed that there was no such pattern, it was all just selective perception."
In the Illuminatus! Trilogy, he expresses the same view: that one can find a numerological significance to anything, provided "sufficient cleverness."
Cultural references 
- The 1998 German film 23, starring August Diehl as computer hacker Karl Koch, tells the real-life story of computer hackers inspired by Wilson's Illuminatus! Trilogy.
- The 2007 film The Number 23, starring Jim Carrey, is the story of a man who becomes obsessed with the number 23 while reading a book of the same title that seems to be about his life.
- Industrial music group Throbbing Gristle recounted in great detail the meeting of Burroughs and Clark and the significance of the number 23 in the ballad "The Old Man Smiled".
- German bitpop band Welle: Erdball recorded the song "23 (C64'er Version)" (released as a B-side to the single "Starfighter F-104G". The lyrics contain lots of references to Wilson's and Shea's Illuminatus! Trilogy.
- "Going loco over 'El Becko'"
- Robert Anton Wilson on the "23 Phenomena"
- Benecke, Mark (2011). "The Numerology of 23". Skeptical Inquirer (Committee for Skeptical Inquiry) 35 (3): 49–53.
- Principia Discordia, pg. 23
- Robert Anton Wilson sees the clustering illusion everywhere, not just 23, Robert Anton Wilson Explains Everything (audiobook), December 2001.
- Wilson, Robert Anton; Robert Shea (1984). The Illuminatus! Trilogy.