Recently, I was reading an old (well, old to today's Internet standards) by John Bethune, "Innocent and Malignant Typos and the Case of Filloux v. Jarvis." (http://www.b2bmemes.com/2011/06/24/innocent-and-malignant-typos-and-filloux-v-jarvis/). He talks about the permanence of typos in printed media. In a journal or newspaper, they can run a correction in the next edition, but it still stands forever in history. When people go back to do research or find the old newspaper in an abandoned garage, they will see that mistake, loud and clear.
While everything lasts forever on the Internet and nothing can ever be entirely erased, it does leave a little more wiggle room is this area. If you accidentally miss a typo in a blog post, online article, or wiki entry, you can easily go back in and edit it. No doubt, some people will have already pointed it out and made your mistake very public, and their remarks will remain. But, you can still go back and correct it. Future readers who visit the web page will see what you meant to say, not what you initially typed.
But this also leads to another idea: Why is proofreading so absent in online publications? With the prevalence of typos out there in the blogging world, why aren't people more careful of what they write? Even though these errors can be fixed easily and change the record, it still makes a bad impression. We all know that spelling is not a sign of intelligence level, but many still perceive it that way and will judge you for a simple mistake that could have been caught early. Many blog hosts come with spell check built in, but even if yours doesn't, you should still be able to copy your work over to Word to use its spell check software. Also, don't doubt the importance of giving your writing a read-through before publishing. Read it backwards, have someone else read it, print it out to look it over, whatever you have to do; just make sure your writing is a proper representation of yourself before you send it out into the world to be judged.