Revealing mistake: As Kevin is flying off the front porch on the toboggan, you can see the small wheels mounted on the bottom of the sled to aid in its "jump". (00:25:30)
Revealing mistake: In the skywalk scene, when McClane pushes the paint stand on the terrorist, it is quite obvious that they use a dummy when the paint stand falls on the terrorist and crushes him. (00:45:05)
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.