Revealing mistake: In the scene where Kim comes home early from the camping trip, Edward is startled and begins stabbing the water bed, thus causing it to spring many leaks. If you actually look at the bed, it isn't jiggling like a water bed should and the water is just spraying out even in places he isn't stabbing.
Revealing mistake: When Edward is leaning in to touch the deceased Inventor's cheek, you can see the Inventor's left eye twitch.
Answer: If the robbery had been successful, Jim would have been able to accuse Edward of robbing the house with his friends backing him up. Jim probably would have kept the car at a friend's house to avoid suspicion. Since the house belongs to Jim's father, nobody would question him going inside. If Jim and his friends tried to rob someone else's house, there would be the chance that somebody would be home or that a burglar alarm could go off and alert the neighbors and the police.