* Erwann Abalea:
Or more commonly because some equipment (a firewall, most of the time) closes the connection at both ends, and the browser retries the connection with a protocol downgrade. Web browsers don't intentionally break the handshake, they try to adapt to various servers+networks environments to get the resource desired by the end user.
They enable server operators to get away with non-compliant behavior. Now they even punish those who actually maintain their web servers by forcing them to implement TLS_FALLBACK_SCSV support. Web browsers are very much too blame for this particular mess.
And even worse, developers now rush in client application changes to send TLS_FALLBACK_SCSV on every handshake, even if they do not perform a browser-style insecure protocol version downgrade. This will make deployment of TLS 1.3 on servers rather difficult.