When i was going down a ladder and my sister when up it, when we collide i begin to head up with her and I told her to stop but we still keep going up until we ended up where I begin which was the top of the ladder. I know this in 15w50b when I was play with my brother when we were both headed down the ladder to get to the end portal.
This is because they reintroduce the player pushing mechanics. So that means players can now push other players places. Including on ladders.