I've been reading a lot of articles about the origin of the internet, namely this one, however, I think there's still a lot of ongoing debate about things of lore like the 404 ERROR. Hence, I'm asking historians about their take on this.
Nothing more mysterious than being between 403 and 405 on the list of status codes in the HTTP (Hypertext Transfer Protocol) standard. The HTTP status codes are 3 digit codes, organised in 5 blocks identified by the first digit, and the particular status code within that group by the last two digits.
The HTTP/1.1 codes were
1xx - Informational: 101 Continue, 101 Switching Protocols
2xx - Successful: 200 OK, 201 Created, 202 Accepted, 203 Non-Authoritative Information, 204 No Content, 205 Reset Content, 206 Partial Content
3xx - Redirection: 300 Multiple Choices, 301 Moved Permanently, 302 Moved Temporarily, 303 See Other, 304 Not Modified, 305 Use Proxy
4xx - Client Error: 400 Bad Request, 401 Unauthorized, 402 Payment Required, 403 Forbidden, 404 Not Found, 405 Method Not Allowed, 406 Not Acceptable, 407 Proxy Authentication Required, 408 Request Timeout, 409 Conflict, 410 Gone, 411 Length Required, 412 Precondition Failed, 413 Request Entity Too Large, 414 Request-URI Too Long, 415 Unsupported Media Type
5xx - Server Error: 500 Internal Server Error, 501 Not Implemented, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout
Codes 301, 302, 404, and 410 are similar. All say that "nothing is here with that Uniform Resource Identifier (URI)". 301 and 302 responses should include the new URI (permanent and temporary, respectively) for the requested resource. 410 says "I think it's gone permanently, but I don't have a new URI for it". 404 simply says "I can't find it, and I don't know why". 301 and 302 are "Redirection" status codes, because they allow the new URI to be used to request the resource. 404 and 410 are "Client Error" codes because the client (e.g., you asking for a web page) is asking for something that doesn't exist.
The different classes of status codes are just numbered 1-5, and the different status codes in each class are just numbered sequentially. Once you know that the client error codes are 400, 401, ..., 415, then "404" ceases to be mysterious.
Reference and reading:
Status code definitions in the HTTP standard 1.1, 1997 - we are now multiple versions later, but this is a good historical view of early HTTP standards:
HTTP/1.0 was similar, but didn't include the 1xx codes: