Continuity mistake: In one of the scenes Hopkins is trying to make what I think is a fish hook or something. He's obviously been in the wilderness for a while so he's not clean shaven but he isn't exactly beardy either. However, during the scene with this fish hook his beard changes from moderate to bushy to moderate again within seconds.
Continuity mistake: When Charles is showing Steve the constellations, the sky is pitch black. Yet in the next scene when Charles goes down to the shore to Bob, the sky has brightened considerably. No indication is given that a good deal of time has passed between these two scenes.