I found a bug concerning the deletion of the "remember me" cookies---empty string value cookies are not removed. I wrote about it on my site. Here is the permalink:
Forget You, I'm Keeping The Cookies, zcrisler.net

For those unwilling to checkout my site, here ya go:

This afternoon I got fed up with my individual archive page template not validating as XHTML 1.1, so I started to investigate. The problem was with the comment form, namely, name="comments_form", is not allowed. The easy work around is to change it to id="comments_form" and alter the Javascript to use getElementById. I made the proper adjustments and got the page to validate. Then I figured I should make the same changes to my comment preview and error templates. However, since these pages use <$MTCommentField$> I was unable to make the necessary changes. I eventually stumbled across The Tweezer’s Edge’s solution for replacing the tag.

Long story short, I found that the default Javascript included with Movable Type v3.11, copyrighted 1996-1997 by Athenia Associates, was not deleting empty cookies. To reproduce the error simply fill out the proper information on the comment form and select to remember it. Post your comment. Check your cookies. You should notice that all three cookies, mtcmtauth, mtcmtemail, and mtcmthome are all set to the values you specified.

Now go to make another comment—your information should be set for you—and delete the URL. Why the URL? Because you can still post without one. Make sure you still have the remember option selected. Post again. Check your cookies. mtcmtauth and mtcmtemail should be set properly. mtcmthome is set to an empty string. This is okay since this value for a cookie may be desirable. However, go to post a 3rd time and select to not remember your info. By default, there is an “onclick� attribute for this option—it will forget you as soon as it is selected. You should see all the fields—“name�, “email�, and “url�—blank. Check your cookies (last time, I promise). You will notice that mtcmtauth and mtcmtemail are properly deleted, but mtcmthome is still there with the value of an empty string.

What happened? Why is it still there? Well, the getCookie function returns the length of the value string or an empty string when the length is zero. The deleteCookie function uses this simple if conditional to verify the cookie exists before attempting to delete it:

if (getCookie(name))

Since getCookie will return an empty string the conditional fails for mtcmthome and it is never actually removed.

A proper solution would involve rewriting getCookie to return only a proper length. However you can simply add an extra part to the conditional in deleteCookie to fix the problem:

if (getCookie(name) || getCookie(name)=='')

That should take care of removing cookies with values of empty strings.