Weird rollback behavior I saw in Windows Installer yesterday

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I saw a very odd rollback behavior in the .NET Framework setup yesterday that I thought I should post in case anyone else runs into something similar. This was the 2nd time I saw it, but the first time in a scenario from a non-Microsoft internal machine. When it first happened I had written it off as a one-off caused by daily builds of the OS, the .NET Framework or something like that. But apparently it is possible to get the machine into this state through released products (possibly beta bits, but still valid in my opinion)

Essentially what happened was the .NET Framework setup proceeded all of the way through installation and then initiated a full rollback. I couldn't find any evidence in the verbose MSI log file that any kind of error had triggered the rollback, and the last section of the log told me that the return code was 0 and that the .NET Framework installation completed successfully. I had to use filemon and regmon (from https://www.sysinternals.com) to diagnose that there was a file in the folder c:\config.msi named *.rbs that was being executed, and then finally I traced it back to the following registry value on the machine:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\Rollback\Scripts

I wasn't able to determine how that value got set, and searching for information about this registry hive in msi.chm and on the internet didn't yield any conclusive information. The closest I came to a possible explanation is in the section named Active (Incomplete) Installations at https://www.microsoft.com/resources/documentation/WindowsServ/2003/all/techref/en-us/Default.asp?url=/resources/documentation/windowsServ/2003/all/techref/en-us/msizap_remarks.asp

What I think may have happened is some other Windows Installer setup (more than likely the .NET Framework itself) got orphaned before it completed, and then this script was left behind in the registry and inadvertantly got launched by future installations. Normally when a Windows Installer setup gets orphaned and you try to run another MSI, it will say that an installation is currently in progress and is suspended and asks if you want to rollback before you continue with the new setup, but for whatever reason we didn't see that in these 2 instances. I'm still hoping to get a more thorough explanation from the Windows Installer team about what is really going on here, and I'll post my findings back here.....