I am running the latest beta and I have only two addons installed. Jaxel's portal and ragtek's navbar manager. I have not been able to recreate it a 3rd time but I moved a forum and it after it was moved there would be a second one with out a parent. If I can do it again ill screenshot or something.
It appeared in the node list in the ACP with the same one and I removed it through there multiple times before it finally vanished. On the front end it appeared like a forum with out a category parent.