When writing and pasting text into a wordpress blog post, I found that some strange code like
was added automatically at the end of the post.
<img style="border: medium none"
First thought was some trojan/virus. After looking deeper into it, I found it's only a Vista Aero(Firefox theme) error.
And that code is nothing but this image:
To fix this issue, either remove Vista Aero or update it to the latest version.