The page /~fred gives information about the user fred.
But if fred does not exist (or if you mistyped the
URL) it still returns a 200 OK http response. For example
% wget http://slashdot.org/~fongrel
Resolving slashdot.org... done.
Connecting to slashdot.org[188.8.131.52]:80... connected.
HTTP request sent, awaiting response... 200 OK
Instead, the response from the server should be 404 Not
Found. The page served can still give the message
about the user not existing.