Audio problem: When Jeremy and James are playing I spy, just as Jeremy says "If you say S, I'm going to kill you!" there is a shot from outside the car and you can see that his lips aren't moving.
Other mistake: When Mattie first gets her kite out, Richard says that they are in the same fjord where Jeremy and James went through the ice. If so, the scenery has changed remarkably in a couple of days. All the blue ice is gone and so are the cliffs, which prevented Jeremy and James driving out. He can't have taken a different route as the fjord is only a mile wide.
