I've recently noticed that I cannot climb ladders on my SMP, where I play on Scale 3. I've jumped to Singleplayer and tested both vines and ladders, where I noticed that the threshold for being able to climb ladders was at 2.7, while the threshold for vines was 3.33. Placing multiple columns of ladders adjacent to eachother also didn't work, so this seems to be a problem with collisions at higher scales.