This slip-up happens when Greg has chased the cat on the roof. He sits down and smokes a cigarette and eventually the leaves in the trough start on fire. He uses his foot to put out the fire and jumps down. Just before he jumps down you'll see a decent sized fire in the trough yet. For the split second that the camera shows where he was sitting after he jumps, you'll notice that the fire is no longer there. Dry leaves must have a way of putting themselves out nowadays.