Revealing mistake: When Alan falls off the ladder you can see the ladder has been set up so it will fall back safely. As the ladder goes back towards the balcony rail, it hits the railing and bounces up. The bottom of the ladder, which is unseen, seems to be planted as it does not move during this. When Alan was climbing up and down the ladder, it wasn't secured with anything at the bottom. Thus the reason for him falling off the ladder.