Oops:

The hack, which affects only Twitter.com and not third-party clients, works by putting a piece of JavaScript code ('onmouseover') into a URL in a tweet. This causes a pop-up message to emerge when someone hovers a cursor over that link. The loophole appears to work in both the redesigned Twitter web interface that was launched on Wednesday and the previous version

The problem has been dealt with, according to Twitter - but boy, that's an embarrassing launch "oops" for them.

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