If you try to crouch/sneak to the edge climbable blocks like vines and ladders, you will fall off them. 

Steps to reproduce:

1) place a column of climbable blocks 

2) climb them and crouch/sneak 

3) approach a side that doesn't have a block near it 

Observed results: you fall off. 

Expected results: crouching/sneaking on climbable blocks should allow the player to stand a small distance beyond the edge of the climbable blocks but also prevent the player from falling off them. 

Additional information: you won't fall off if you're crouching over the edge of the top of a ladder, but you will if you're crouching on top of vines.