Error: MMC has detected an error in a snap-in. It is recommended that you shut down and restart MMC.

When you add a Windows Server 2008 Server in your Windows Server 2003 domain, you have to update the schema. You may notice that when you use Active Directory Users and Computers on your Windows Server 2003 Domain Controller, you get the following error:

MMC has detected an error in a snap-in. It is recommended that you shut down and restart MMC.

According to Microsoft “This problem occurs because the Adsiedit.dll module uses an uninitialized pointer. When the system frees this pointer, some random data is freed. Therefore, an access violation may occur.”

You can download the hotfix from Microsoft to fix the problem. Check out the KB article 946549 for more details.