During your journey of Office 365 nirvana, you may decide to install DirSync (or, as the setup window title more verbosely describes it, "Microsoft Online Services Directory Synchronization"). So, you spin up a new Windows Server 2008 R2 VM, join it to your domain, and, start the setup.
And then it happens.
ErrorCoexistence MSI installation failed. msiexec returned 1603 Unable to uninstall the Microsoft Online Services Directory Synchronization tool. Use the Control Panel to remove the Directory Synchronization tool.
What the heck?
Oh, you must have forgotten to install the .NET Framework 3.5.1 Features feature. How silly of you.
You know what to do.