Or, "Post in haste, repent at leisure." One of the most useful pieces of advice I was ever given about the online world was back when I was a wee newbie; I was told not to post in anger. Write it if I must, but then leave it. For at least ten minutes, preferably overnight. Come back and look at it when calmed down, and ask myself, "Do I really want to send that?"
This was in the days before Google, when dinosaurs roamed the earth and Alta Vista ruled the search engines. And when a search engine company called Deja News was causing consternation by archiving and indexing the casual conversation on a system called USENET, making it possible to retrieve and read posts that had been made years earlier in the expectation that what was said was ephemeral, the online equivalent of spoken words, gone once the conversation was over. There had always been small specialist archives, but Deja News made it possible for someone to call up the entirety of a person's posting history, see what they said on any subject, and to whom they said it. Casual, semi-private conversation was no longer either casual or semi-private.
Why does this matter? Because the net has a funny effect on people. It dehumanises the social transaction, makes it easy to forget that those are real people on the other end of the wire. Or it simply insulates people from the consequences of their actions. And thus it becomes easy to say things that you'd never say to someone's face, or that you'd never say in front of a camera recording your words for posterity. Things you might find embarrassing to have quoted back to you at a later date.
People have long memories, and it's neither nice nor wise to insult them unnecessarily. But machines have longer memories still, and back when dinosaurs roamed the earth, we were starting to understand what that meant. What goes on the net, stays on the net. You cannot take back your words once they've been said. They remain frozen in time, on some storage medium somewhere, and you can never, ever be quite sure that they won't return to haunt you. With datestamps and IP addresses to prove that yes, it was you who said them.
You can take down the webpage or blog post you wrote when you were drunk and angry, but Google cache will continue to show it to the world for a few hours or days or weeks. And archive.org may well have it in permanent archives. You can ask them to remove it, and they will, eventually. In the meantime, people will have been making screen shots. If you comment on someone else's blog, you're just going to have to hope that it's one that allows you to delete your own posts, and that it's not set to email copies of comments to the blog owner.
You can try to keep things private. Locked LiveJournal posts? Well, there was that episode when someone offered a useful service that just happened to ask people to login using their LJ password. And a couple of months later, the owners had enormous fun publicly posting the contents of locked posts they'd gained access to because a friend of the poster had subscribed to this service. Private bulletin boards get hacked. Oh, and that private mailing list may be exposed when some virus scoops up the contents of someone's hard drive, and starts emailing it to random addresses. I've been on the receiving end of such virus-forwarded emails, and it took a formal complaint to the infectee's ISP to get them to clean up their computer. Private tantrums are much, much better than public, but you can't assume that they'll stay private.
None of this means that you must be a saint at all times, never saying anything controversial or nasty, not even in passive-aggressive fashion or in a private forum. After all, I'm not known for my shy and retiring nature online. You'll go nuts, and you'll be boring. And people will forgive a lot, if you make an honest apology for something you regret in the cold light of morning. But own your words. Because they will surely own you, come the day you feel a need to deny them.
Welcome to the Panopticon, little author. Where there are a million cameras watching you, each and every one of them recording you for posterity. Try not to show your knickers in public too often.
[No, this isn't aimed at anyone in particular. I've been meaning to write something along these lines for months, wrote the first draft over a month ago, and have been sitting on it ever since, waiting for the opportunity to post it on a quiet day when nobody was going to take it personally. I think this roughly translates as "when hell freezes over", so it's going up today.]