428 Precondition Required (RFC 6585)
This is an educational reference page about HTTP 428 Precondition Required. The page itself is served as 200 OK so it can be indexed as HTTP documentation.
The origin server requires the request to be conditional. Intended to prevent the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict.