I have HP MSR routers that allow you to tag and untag the voice VLAN on the same port, but I can't think of any Cisco, Procurve or Juniper switch I have ever worked on that would allow that. I think 3COM switches do some weird stuff like this.
It means the controller will be seeing two copies of every packet. I doubt the controller will read the tagged ones if its own interface is set to untagged, but if it did, you could get some weird stuff happening.
I'd call it a security issue, personally, but those hardware controllers do some strange things when they reboot or during an upgrade so maybe he's done what he's done for a reason.