A general system error occurred: unable to save the snapshot file

For the last hour or so I have been attempting to take a VM snapshot on my ESXi 5 host at home but keep encountering the f0llowing error:

A general system error occurred: unable to save the snapshot file

I have tried searching for possible causes and solutions but all I can find are suggestions that there isn’t enough disk space.  This isn’t the case and I’ve tried the usual (restart the guest, restart the host, move the host to another datastore, etc).  After playing around with various settings it turns out that this was caused by an ISO image mounted on the guest CD/DVD drive!  After removing the ISO file I was able to take the snapshot without error.