Just came across with a funny post. It deserves a share. Have fun.
"How to use a Domain Controller as an Exchange 2013 DAG Witness Server (Don’t)If you’re setting up Exchange 2013 in a lab, you may have a requirement to use a domain controller as a File Share Witness (FSW) host.
Obligatory disclaimer: While this is definitely not recommended practise in a production environment, and may have undesirable results, it will probably work. I highly doubt Microsoft support this, though I have been unable to find any concrete evidence on it (frankly, it’s such terrible practise they shouldn’t have to publicise advice against it). If you can come up with a method to avoid following this guide in production, I implore you to do so.
The File Share Witness is used as the winning vote when your DAG has an even number of hosts – the same principle as the quorum drive in a Failover Cluster. This is a file server that all DAG members can read and write (but cannot be a DAG member for obvious reasons).
If you just try to create a DAG and use a domain controller as a FSW, you’ll get this error message:
Read the rest here.