Earlier this morning, Twitter was apparently hacked, and a virus was allegedly spread via shortened links. Everyone also had zero followers and were following zero people.
This supposedly only happened to the now-labelled "Old Twitter." Ironically, clicking the "Twitter got hacked" trending topic showed Twitter's own promoted tweet, touting New Twitter.
Twitter has of yet to announce official news on the hack or any damage done. Will update as news unfold.
UPDATE: The trending topic "Twitter got hacked" vanished. Given that is was sitting strong in the second slot, the most likely scenario is that Twitter axed the TT for PR purposes.
UPDATE 2: Pop-up windows were the major damage-doer of the hack, and were activated simply by mousing over a link—you didn't even need to click them.
UPDATE 3: Twitter Safety has announced the vulnerability has been fully patched.
UPDATE 4: Twitter has published a blog post outlining the situation in full detail. Their short story:
This morning at 2:54 am PDT Twitter was notified of a security exploit that surfaced about a half hour before that, and we immediately went to work on fixing it. By 7:00 am PDT, the primary issue was solved. And, by 9:15 am PDT, a more minor but related issue tied to hovercards was also fixed.
Check out their blog for the full explanation.