There’s a little bug in the latest version or it might be a conflict with a plugin. I edit posts in HTML editor, and when I select a portion of text and add a link, the entire text gets selected and the code jumps right to the first line of the editor.
It looks something like this after hitting insert link:
<a href="http://www.link.com" /></a>
Text of the article with the word that was supposed to be <strong>link</strong>. Rest of the article.
So I have to be careful, to move the link to its place by hand. It also happens with insert media command, when I have to move the place where a picture should be from the first line to the proper line.