– hover the menu titles, and you get a box saying "404 page not found".
The reason is that the script adds an IFRAME element (meant to include other pages in the current page) pointing to a missing file. This element is meant to be behind the menu. But in Opera (and older versions of Mozilla and IE) an IFRAME can not be hidden behind parts of the page because it is like a separate but embedded window. The mysterious question is: why does the script's author want to hide an inline frame with a "not found" page behind the menu? It serves no other purpose than obscuring the menu in browsers where IFRAMEs are on the window level, as far as I can see.