Today I ran into this error when trying to print a page using the print feature of IE6. The error only occurred on the site I had developed, so it wasn't an issue with the user's machine. Apparently, the issue was that I had set the id of an img element to "tags". I changed the id to "tagsImg" and now the error is gone.
Apparently id="tags" is a reserved word in IE6 that has something to do with the print feature. Read this blog post and the comments for more info: http://meyerweb.com/eric/thoughts/2005/08/26/when-printing-kills/