431 Request Header Fields Too Large

The headers did not fit — almost always a cookie that grew too much.

Also from RFC 6585: the server refuses the request because the headers went over the limit — the whole set or one specific field. The body should say which excess it was.

The classic case is the Cookie that fattens for years until it bursts some server's buffer along the way. That is why 431 error pages tend to suggest the only fix within the user's reach: clearing the site's cookies.

HTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/html
Connection: close

<h1>Cookie header too large</h1>

The request died under the weight of its own headers.