Under certain circumstances, the STSADM MergeContentDB command may fail in Windows SharePoint Services 3.0. These circumstances include combinations of significant site collection size, user traffic, and SQL Server load. When the STSADM MergeContentDB command fails, both the source and destination databases can be corrupted. http://support.microsoft.com/default.aspx?scid=kb;EN-US;969242…

If your clients can afford it or you are moving Sites (Webs) within a Site Collection you should be using SharePoint Site Migration Manager by metalogix.

-Ivan