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A few years ago, when I was a younger but possibly more idiotic man than I am today, I posted something stupid on the internet. I just didn't think. Sheer embarrassment prevents me from revealing exactly what it was, but suffice to say it didn't make me look big, or indeed clever. Worse: it's still there. I can't get rid of it. I live in fear of someone stumbling across it and confronting me about it. It'll niggle in the back of my mind until either I die, or the internet dies, and frankly I think the internet's going to win.

Did I learn from my experience? Of course not. A couple of months ago I took a photograph of my computer and posted it on Twitter - there was a good reason for this, I promise you - before I suddenly realised that there was a post-it note stuck on my computer that contained my credit card number, the expiry date and the 3-digit security code. All of which were now visible to the world.

And even if I deleted the photo - which of course I did - there was no way of knowing who might have reposted it, copied it, or the number of servers around the world that now contained a pristine copy of my credit card details. The genie was out of the bottle, so I had to make a slightly embarrassed call to my credit card company.

People think of the net as like a public noticeboard, but that's really not the best analogy. If you pen a piece of soppy poetry and make the questionable decision to pin it up in your local library, you can take it down when you realise your catastrophic error. But the consequences can be more far-reaching if you post such a thing online. Material lingers, duplicates and spreads, and your verse can quickly become a self-replicating personal catastrophe as the world is loudly informed that you "wuv your ickle kitten".

Even deleted web pages can be archived by search engines. For example, when you do a Google search, looked for the "Cached" link next to each result - it can often reveal an older version of each page. Services such as FreezePage, meanwhile, exist precisely to capture transient moments on the web and have been used to cause embarrassment. Then there's archive.org - a colossal repository of web content from yesteryear - and Google Groups, which archives all the discussions that have ever taken place on Usenet.

Ten years ago Usenet was the premier online discussion forum, but no-one ever imagined that their online bickering would be unearthed a decade later. What about ill-thought out blog posts that, on reflection, you decided to delete? Automated content collectors may well have grabbed those and sent them whizzing around the net while you're asleep. Of course, none of these services set out to undermine us - they can be fantastically useful - but they can easily double up as sources of shame and regret.

Social networking sites make things even more problematic. They constantly badger us to share information about ourselves and others, and in many cases we're unaware that these things are visible until someone contacts us and says "er, did you know that there's a picture of you being sick at a wedding on Facebook?" Cue a personal plea to the person who uploaded it; but there's no guarantee they'll remove it.

And you can unwittingly do this kind of thing yourself without realising, too. Last.fm is a good example: it's a service that shows the world a list of what you've been listening to on your iPod or your computer, and can provide a fascinating data set about your listening habits. But do you want the world to know that you spent one Saturday afternoon listening to The Wombles on repeat? Probably not.

There are many who'll post things online while breezily saying that they have nothing to hide. But they may change their tune when a potential employer discovers that they once built an online shrine to a member of Girls Aloud, for example. It's probably not mentioned in their CV, but, thanks to the internet, it may as well have been.

Because the internet's a wild beast you can't control: stuff you're desperately looking for might be impossible to find, but the things you dearly wish would disappear forever can remain stubbornly, embarrassingly visible. Fortunately, most of the information we put out there is frivolous and unimportant - but it's always good to think twice before clicking "send".

If you'd like to learn more about etiquette and libel on social networks, have a look at our new Social Media Basics course.

Rhodri Marsden is a writer and musician who regularly details his fascination and exasperation with modern technology and the internet for both The Independent and ´óÏó´«Ã½ 6Music.