The 404 error message, that is. According to linguist and prolific author David Crystal, "404" was the number of the room at CERN, in Switzerland, where the World Wide Web was developed. Update: Or not. See Tomas's comment, below.
I discovered that tidbit and many others in Crystal's Words Words Words, whose chapter titles all start with "word"--Wordsmithery, Wordbuilding, Wordrisks, Wordbirths, Worddeaths, and so on.
Here's a link to some classic 404 error messages as well as some creative variations.
And in news otherwise unrelated except that it involves a three-digit number beginning with "4," perhaps you've heard that California's Compassionate Use Act, passed by the voters in 1996, allows the medical dispensation of marijuana. In 2003, the State Legislature modified the act with a piece of legislation formally known as--wait for it--SB 420. Yes, "420," as in ... 420. Sheer coincidence? Well, the bill's author was former State Senator John Vasconcellos, who represented the Silicon Valley area for eons and whose obituary will almost certainly include his role as the godfather of California's self-esteem movement. High technology, high self-esteem ... I rest my case.
I read about SB 420 in this fascinating New Yorker article by David Samuels about the weird world of medical marijuana in the Golden State. In additional to numerological trivia, the article shares some marijuana nomenclature, e.g.: Sour Diesel, Bubba Kush, AK Mist, Purple Urkel, Jedi, and Gush ("a robust mixture of Goo—a lighter, giddier high—and Kush").