Very often, companies do not do enough to test their backups adequately. Just because you have tested a recovery procedure before and you are not showing any errors in your backup system, does not necessarily mean that you can successfully recover. First, there is the obvious. If you do not practice procedures, you are more likely to make mistakes. More importantly, things change. Systems get replaced, upgraded, patched, and have configuration changes.
One example of this is evident with Windows 2008 system state. With Windows 2008, Microsoft has replaced the older NT Backup with Windows Server Backup. As this is a different tool, it requires different procedures for recovery. This may not be a big deal for most Windows servers, but it is critical to pay attention to system state recovery when it comes to Active Directory.
The change from NT Backup to Windows Server Backup is a pretty significant one that you most likely were aware of. Something more subtle that may affect your procedures is removal of boot.ini. This is something completely outside of the backup system that could affect your recovery. Without prior knowledge of this change, there could be a significant delay in recovery of Active Directory.