Maybe this one's already been documented-- I should have thought of this as something to avoid already.
On Quills 1.7 on Plone 3.3.4, if you have a category with the same name as a folder in the site, Quills gets confused when rendering the topic.
For example, I had a folder named 'office' and a tag named 'office.' Clicking any other tags in the Quills tag cloud rendered the 'foo/bar/topics/tag' structure fine, but clicking the 'office' tag threw an error.
My 30-second workaround was to rename the category keyword.