When I would go to edit a node, I would notice that instead of showing nice looking tabs, the links that were supposed to look like they were in tabs were all squished together. Same thing happened for the tabs in the user's profile. What's weird is that I could swear that they used to render as expected. Anyway I copied tabs.css from the zen theme into the css folder of my subtheme (or course adding it to the *.info file, too). Now the tabs look great again. Weird that Drupal wasn't using this Zen stylesheet.