I think I've found a bug in version 9.2. If you have a multi-resource page, and you edit the single occurance on a multi-occuring event, it moves that single occurance from your resource to the unassigned resource (-999). I've been able to duplicate the error on several databases.
Does anyone know if this has been corrected in subsequent versions?
I found the problem. These were created in a single view calendar, and didn't have the same setup in the multi-view.