During the scene when Mary and the children are picking up the nursery, you see a rocking horse which is standing upright. A short time later, however, the horse is laying on it's side and Mary stands it up as she walks past it.
Use your imagination! There are 101 ways that horse could have fallen over again, I don't think this is a slipup. I think you should have read the instructions before you enters this one, they ask is there any explantion for the slipup - in this case? YES!