UPDATE 19/01/2011 13:14: The Metropolitan Police has blamed a leaked training document, and not the since-deleted poorly-timed Tweet from ASOS, for the panic. Full details available in our updated story.
UPDATE 19/01/2011 12:52: Mark White, of Sky News, is claiming that the Metropolitan Police carried out a "routine firearms training exercise" that resulted in the sudden mass panic over a non-existent shooting in Oxford Street. As this contradicts the Met's statement that there have been no reports of gunfire, it appears that not even the police know what's happening.
The power of microblogging service Twitter for rapid real-time dissemination of gibberish was proven today when a ill-worded tweet about a photography expedition turned into panicked rumours of a fatal shooting in London's Oxford Street.
The original message, posted by fashion assistant Candice Bailey, since deleted but saved on Twitter archiving service EquisiteTweets, read "Street style shooting in Oxford Circus for ASOS and Diet Coke. Let me know if you're around!!"
Sadly, this was retweeted, remixed, and taken out of context in a mass game of Chinese Tweets that ended up seeming to refer to a street proper gun-style shooting - resulting in thousands of confused London-based Twitter users posting increasingly urgent missives to find out if friends and family are safe from an unknown gunman.
The messages, which are still pouring in to the service, have lead some users to get in touch with the Metropolitan Police, who have confirmed that there have been no reports of gunfire in the Oxford Street area.
The power of Twitter for instant real-time updates is undeniable, but as with all sources of information, one thing is key: always verify what you're dealing with a reliable source before going off half-cocked.