Not Found
[Instead of the regular xkcd site layout, just a white page that states on top center:] 404 Not Found
[Page-wide divider line]
[Below that in a smaller font:] nginx
It thus leaves the xkcd page.

There is no image for xkcd comic numbered 404. Randall did, however, not skip a day!
The previous comic 403: Convincing Pickup Line was released Monday March 31st 2008, and he put the next comic, 405: Journal 3 , up as scheduled on Wednesday April 2nd 2008, which leads some to see the 404 as an April Fools' Day joke released in between, as comic 404: Not Found released on Tuesday April 1st 2008.
This was thus the first April Fools' comic released by Randall (not counting the unnumbered Syndication released the previous year). The next (and first real) April Fools' Day comic was not released until 880: Headache on Friday April 1st 2011, although Randall did make other jokes in the years in between.
"404" is the HTTP Response Code for " Not Found ." Randall deliberately skipped comic number 404 in xkcd. Therefore, when people go to xkcd 404 , they get a "404 Not Found" error page. (This does not work in all browsers. In newer versions of Internet Explorer, a message about the link being broken occurs without the 404 code.)
Randall has stated that he considers 404 an official, actual comic, albeit a rather avant-garde one , and that for a time he made it possible to find it using the "random" button on xkcd.com. He once again displays this in the 2018 April Fools' joke, 1975: Right Click , where you can access all of his comics up to Right Click from the right click menu, and the menu includes this one specifically due to the strange "title"!
It may seem like a random coincidence that xkcd 403 came out the day before April 1st 2008. And for sure it must have been by chance that it would come out close to that date when Randall began posting on xkcd. But when Randall noticed this fact, at some point prior to that date, he would, however, have had a chance to influence the release date. In November 2007, less than half a year before this April 1st, he released the 1337 series over five consecutive days. That would have moved the release date of 403 from Friday the 4th of April to the Monday it was actually released - making it possible to skip comic 404 as if it came out on April 1st without skipping a comic on a normal release day. There was at least one other series in 2007 to use all five days of a week ( Choices ), so maybe he had this planned for a long time?
It is perhaps an interesting point that the very next comic, 405: Journal 3 , includes the line "So, you found me after all."
In 1969: Not Available , the error message 404 is referenced in the caption.
This is featured in a few comics to come, including 1975: Right Click (in File > Open > C:\ > Bookmarks/ > Comics > comic num 404).
[Instead of the regular xkcd site layout, just a white page that states on top center:] 404 Not Found
[Page-wide divider line]
[Below that in a smaller font:] nginx
It thus leaves the xkcd page.
