Revealing mistake: Mason brings his unnamed son to the lake and acts like he is going to drown him. The baby is noticeably a doll in all but the scene where he kisses it. It is obvious from the way he is carrying it as well as the lack of movement.
Continuity mistake: Marty pulls his boat into a cove where a church sermon is happening. When we first see the sermon the sun is coming from the right. In the next scene the sun is behind Marty. It switches again when he pulls into the cove.