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 sa pamamagitan ng kicking the shifter away and rubbing the rope on the knife, and stood up, WITHOUT freeing his feet. Then the susunod time we see his feet they aren't tied. Did I miss something, or is this just a goof on the show's part?