Let’s face it, we all make mistakes. I learned the hard way that editing a live site without testing it not such a great idea!
I did something which completely messed up the blog – just something like not closing a comment tag, so that meant that everything between that comment and the next comment close just disappeared. And boy, did I panic when I realised! So, my first tip for all bloggers (and site owners) is: create a test blog. You can do this very easily using a free service like Blogger and WordPress. And then you can play around with it and modify your html before releasing changes on your real blog.