On the night of Rob's wedding, he and Bram intervene when Gordon's men are about to kill Riddon, the leader of the Mutates, and set him free. Later, after Gordon overthrows the Norm leader, Rob and Bram escape and soon run into a group of Mutates led by Riddon, who says he remembers them because they gave him his life "a few days ago". In fact, as the events and time line in the film make clear, only a few hours had passed since their first encounter earlier that same night.
When the Mutates raid the Norm caves to seize women, the first captive they take is Ruth, who has been imprisoned in a cage and is wearing a long, flowing white dress. But when the Mutates are seen a short time later leading the group of captured women back to their land, Ruth is suddenly wearing a shorter dress of a completely different style and obviously darker color. Since the Mutates were doing battle with the Up River Men while they were kidnapping the women, it is highly unlikely Ruth would have been given time to change clothes before leaving the caves.
When Gordon's man throws a rock at Ruth (to stop her from running off to warn her tribe about Gordon's coming attack), the rock hits the wall behind her, bounces off, and misses her entirely, but she collapses unconscious anyway.