When creating custom search scopes within MOSS 2007, be wary of mixing 'shared' search scopes and 'local' search scopes within a single display group. If you create a search scope at the site collection level (via the Site Settings > Site Collection Administration > Search Scopes menu), the scope will inherently be local. Note the absence of a check-mark in the 'Shared' column when viewing the site collection scope administrative page ([Site Collection URL]/_layouts/viewscopes.aspx?mode=site). The default 'People' and 'All Sites' search scopes are shared. Combining both a local and shared search scope into a single display will generate the following error within the search results if a user selects a combination of the 2:

Your search cannot be completed because of a service error. Try your search again or contact your administrator for more information.

To avoid this issue, either define your new search scope as shared by creating it within the SSP scope admin page ([SSP URL]/ssp/admin/_layouts/viewscopesssp.aspx?mode=ssp) rather than the site collection scope admin page. Or create a custom search display group that contains ONLY shared or ONLY local search scopes, so users won't be able to select both.