Last week, a customer came to me with a security trimming issue they were experiencing. Users were seeing search results that they didn’t have permission to see. Fortunately, they were denied access when they tried to access the information from the link in the search results.
I had personally not experienced it but it is documented here.
Items that are associated with a "keyword best bet" in SharePoint Server 2007 are displayed in the search results even though a user does not have permissions to access the items