58. The web is not meant for invalidating data.
Server should not be able to keep clients' state, otherwise
they wont scale well.
That's why long-polling and endless connections haven't
had big success dealing with caching.
60. HTTP's cache fails when dealing with really dynamic
pages, because consumers will always have to hit the
origin server, although a part of the page would be
cacheable ( header and footer, for example )