Creating a page with several different modal YUI panels (popup “windows”). Some of the modals were showing up behind the overlay that shades and disables the rest of the page.
When I looked at the DOM, it turns out that I didn’t properly close some tags, so some of the modals were actually inside the other modals. Any time a YUI panel isn’t a child of the body element, that can cause problems with modality. Fixing the closing tags fixed it.