JSI Tip 9744. The Ntbackup.exe operation may break the differential backup chain of a SQL Server database and may invalidate the differential backups when you perform a snapshot backup of the database?

Microsoft Knowledge Base Article 903643 contains the following introduction:

The Ntbackup.exe utility uses the Virtual Device Interface (VDI) to perform a snapshot backup of a Microsoft SQL Server 2000 database. When you use this utility, the operation may break the differential backup chain of the SQL Server database and invalidate the differential backups.