Errors / HTTP errors

400 Bad Request

The server could not understand or accept what the browser sent.

The page does not load, or the action behind the request (often a form) silently fails.

Seeing it on someone else’s site?

• Clearing the site's cookies fixes it surprisingly often: a cookie grew too large or got corrupted.
• Check the address for stray characters if you typed or pasted it.

Why it happens, if it is your site

• Cookies or headers too large for the server, typically from cookies piling up on one domain.
• A malformed address: unencoded characters, a broken query string.
• The page sent data in a shape the API rejects: a missing field, a wrong type, invalid JSON.

How to fix it

1. For many visitors at once, look at what the page sends; the response body usually names the field.
2. For some visitors only, look at cookie size on the domain and the server's header size limit.

Whose problem it is

Either the site's own code or a third party's, depending on where it comes from.