404 Girl

From The LURKMORE Wiki

A meme related to Yotsuba, the little green haired girl of Yotsuba&. Since its early days, 4chan has been a nucleus for fandoms of unusual or "niche" anime, and the works of Kiyohiko Azuma have always been popular there. While Azumanga Daioh is the most famous of his works, 4chan has always held a soft spot for Yotsuba& and since the main character, Yotsuba, is another way of saying 4chan ("Yotsu" = four, "ba" = "chan" = "leaves") she became 4chan's mascot and is featured prominently in artwork related to the website. This worked as a sort of promotion for the manga, and since then Yotsuba's innocence and childish charm have earned her the endearing love of all 4chan, such that despite Anonymous' affinity for lolis, posting Yotsuba porn is considered taboo, and her series is never to be bashed. However, the amount of artwork related to Yotsuba has declined since the early days since updates to the main page are rare, and that would normally be where most people get exposed to her.

Nowadays, the only common place to see Yotsuba are images that appear when one gets an HTTP 404 Error message, which occurs when a thread or image is deleted and one tries to access it. Legend has it that newfags who were unaware of 4chan's history started asking /a/ who "404 Girl" was, though this origin is likely a myth since not even the dullest Gaiafag would be so ignorant as to not know Yotsuba and if such people existed, the meme would have appeared first on /b/ where the 404 images are far more common.