The bug

If you are being pushed by flowing water, such as having the block adjacent to the ladder being flowing water, and attempt to climb a ladder (and presumably vines as well), you will not be able to climb the ladder until you fully exit the flowing water.

This is especially bad in the case where the water in a waterlogged ladder is considered to be pushing the player, creating an unclimbable ladder, as in the 2nd screenshot.

Ideally, being pushed into the ladder or other solid area while in the climbing area of the ladder would cause you to climb the ladder just as the player attempting to use a ladder normally would, this would allow it to be used as a mob elevator or potentially as a trap of some sort.