Missing document libraries after stsadm import

Once upon a time, I was tasked to move a big SharePoint site (a site with lots of sub sites, with a total size of 9 GB) into a site collection. I did this by exporting the top-level site in our production server using "stsadm -o export" and importing it back with "stsadm -o import" into an empty site collection. After I imported the sites into the new site collection, I found document libraries were missing in many of the sites, and not all sites were created successfully in the new site collection. This puzzled me because the same export and import worked in our test environment.

After a few more tries, I found that the problem was with the export itself. If users are still making updates in the source site when the export is running, the sites will not be imported successfully later. Sometimes, the export itself will fail in the middle, as stsadm cannot find certain files that is being uploaded/deleted by users.