This happened to someone, an officer, in my alliance. They attempted to join with their 7r3 silk and two others for the lane/minis. Removed silk and added overseer and was able to join. Silk was on the war defense map 2 tiers below us. This was a T1 war. Because we were in T1 we cannot even so much as view the challenger map, therefore the defender could not be removed from the challenger map and this could not be used as an attacker. Seems like a bug that should be addressed. Only quest they had active was winter of woe and they were not using silk for it.