Well, it's when Jack spends the night at their house for the first time. Jack's roll away bed falls apart, and he decides to sneak into the bedroom.

You see him get up, and start walking to the bedroom. We also see the skateboard which he "trips" on. But, if you didn't notice, it was a really cheap fall. He was flying in the air before he even touched the skateboard.

Yea...it's kinda stupid, but I think he could have "fallen" a little more realistically.