This can happen, eg, if the event Farmers Seek the Dahan for Aid (the next normal Ravage becomes a Build) occurs on a turn when there is no card in the Ravage slot, and on the next turn the event Urban Development (during the next normal Ravage, each City does +2 damage) occurs. Because of Farmers Seek the Dahan for Aid, the Ravage is skipped, so Urban Development remains active until the next turn.