One particularly unhelpful way to think about the internet is to liken it to a public noticeboard. If you pen a piece of sickeningly soppy poetry and make the questionable decision to pin it up in public, you can always take it down as soon as you realise your error. But the internet doesn't work that way. Your embarrassing verse can become a self-replicating personal catastrophe, spread around thousands of hard disks as a reminder to the world that you "wuv your ickle kitten".
We've all posted things online without thinking, but you really need luck on your side to get the genie back in the bottle. You can find deleted web pages in the cache of search engines like Google; the Wayback Machine at archive.org is a repository of web content from yesteryear that we'd rather we hadn't created; Usenet, once a hive of opinionated discussion, has all its posts archived at Google Groups to remind you of the pointless bickering you once induldged in; and snippets of blog posts you wish you hadn't made can circulate around the net in RSS feeds long after the blog post itself has been deleted. Indeed, Nokia blogged a little too hastily about the launch of their mobile app store last week and then tried unsuccesfully to delete all trace of it, which at least goes to show that we can all make mistakes. Of course, none of these web services set out to undermine us – they can be fantastically useful resources – but they can double up as sources of shame and regret.
Social networking sites, with their constant badgering to share our thoughts and images with the world, can make it particularly hard for us to purge unwanted material from their system. Researchers at Cambridge University have long been critical of Facebook's policies that hoard our data against our will, and they noticed a few days ago that personal photographs still seem to hang around on Facebook's servers long after we've tried to delete them. More worrying, perhaps, is the ability to use sites like tweleted.com to easily browse Twitter posts that users have accidentally made and then deliberately tried to delete. It's always worth keeping in mind when publishing online that the internet's a wild beast you can't control: stuff you're desperately looking for will be impossible to find, while the things you dearly wish would disappear forever will be stubbornly, embarrassingly visible.
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Currently under discussion: Should Wikipedia have banned the Church of Scientology?