Ever wonder why links you find via Twitter don’t show up in your
browser history and aren’t suggested by autocomplete in the url
bar? The t.co link shortener serves known browser user agents an
(instead of the standard
Location header) so that Twitter can
stake itself out as the referrer when coming from third-party
clients. This confuses Safari.
Keeps your history from filling up with indistinguishable untitled t.co URLs.