Happened in Inaugural Wagn Circle. Mike went to create *content rule for Wagn Circle cards. Underneath this is stored in Wagn Circle+*type+*structure. We had given him explicit permission to create +*content cards.
However, whenever you create a plus card (eg A+B), then wagn automatically creates the parts (eg A and B). If you don't have permission to create the card, KABLOOEY!
In this case, there was no card for the Set "Wagn Circle+*type" (it was a virtual card). Mike didn't have permission to create this card, so it broke in the process.