Web sites do not "crash".
Web servers are configured to stop accepting new visitors if the requests in the request queue exceeds a predetermined number. When this limit is reached, an error message is displayed.
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.