Web sites não "travam".
Os servidores da Web são configurados para parar de aceitar novos visitantes se as solicitações na fila de solicitações excederem um número predeterminado. Quando esse limite é atingido, uma mensagem de erro é exibida.
503 Service Unavailable
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.