Are you all of a sudden seeing zillions of messages in the event log with either event id 8tvn or 8nhi? The associated text reads something like:
Unable to retrieve parent node for item at: /SomeSite/SomeSubSite/Pages/Default.aspx of type: Area. This may be expected if changes have been made as the System Account, or if this is a POST request, request type: GET
Unable to retrieve parent node for item at: /SomeSite/Pages/SomePage.aspx of type: Page. This may be expected if changes have been made as the System Account, or if this is a POST request, request type: GET
Check and see if you've got more than 50 sites or pages underneath "SomeSite". If so, then you're probably bumping up against the default limit of 50 supported by the MOSS navigation infrastructure and you're getting errors when page/site number 51 and above is accessed. (At least that's what happening on the site I'm working on.)
To make the error go away, you probably need to bump up the value of "DynamicChildLimit" in the relevant NavSiteMapProviderdefinition in web.config. (Set it to zero if you don't want any limit.)
Alternatively, you can define additional NavSiteMapProviders (overriding the default value of "DynamicChildLimit") and reference them in your master page (or whatever is appropriate to your site).
Here are a couple of links to more information on MOSS navigation: