MSI Installer fails with error: Assembly Install [GAC] at RemoveDirectoryAndChildren, line 393

I created an installer using Wix toolset which worked fine, until I opened 2 instances of setup.exe. The installation fails to install an assembly into the GAC:

"Error 1935. An error occurred during the installation of an assembly 'MyAssembly,version="3.2.xxxx.xxxxx",culture="neutral",publicKeyToken="xxxxxxxxxxx",processorArchitecture="MSIL".  Please refer to Help and Support for more information. HRESULT: 0x80131043. assembly interface: IAssemblyCacheItem, function: Commit, component: {xxxxxxxx-xxxx-xxxx-xxxxxxxxxxxx}
Assembly Install:  Failing with hr=80070005 at RemoveDirectoryAndChildren, line 393".

I think an MSI Transaction remained open, locking up a directory with a corrupt ACL.

Rebooting fixed this problem once, however I ran "Setup.exe" with "Setup.exe /?" open at the same time.

So, which folder is locked up? It's not one from my application. Is it a temp folder created during the installation of a dll into the GAC?

It turns out to be a confluence of errors (on my part):

1. A transaction was open from a system change that had not been rebooted. The installer recovered once, but not twice.
2. The build order was running the previously built msi.
3. The actual error was a misconfigured Component attribute.