In Skin, how did Sam get his feet untied?

When the shifter tied him up, he tied his hands and feet. Then Sam got his hands free bởi kicking the shifter away and rubbing the rope on the knife, and stood up, WITHOUT freeing his feet. Then the tiếp theo time we see his feet they aren't tied. Did I miss something, hoặc is this just a goof on the show's part?