Summary pretty much sums up the problem. Cows can climb ladders right now. Which last time I checked they shouldn't be able to do this.
Environment
I don't think this is necessarily important for this kind of bug. However I am running windows, and java is up to date.
"Any mob can climb a ladder in the same way a player does: by pushing against it. Mobs are not smart enough to use ladders deliberately — they won't stay on a ladder to get somewhere, but a ladder directly in their straight-line path (or if they turn in the process) lets them climb up."