I've found what appears to be an html recursion bug in the Category List widget. Steps to reproduce:

1. Set up a list of categories in your blog
2. Apply the Category List widget to the Submenu Container
3. In the Category List widget, set one category as "excluded"

In the rendered page's source, the excluded category appears to leave behind a closing 'list tag' with no opening tag.

OR:
1. Set up a list of categories in your blog
2. Set one category as a subcategory of another (i.e. MainCat, MainCat\SubCat)
3. Apply the Category List widget to the Submenu Container
4. In the Category List widget, set MainCat as "excluded"

This scenario causes the same extra tag, so it appears to be the hidden category that causes it.

Interestingly, this doesn't seem to happen when the category list widget is used in the sidebar container.