That's actually a very interesting example. If
@clearstream's position is correct, then the player literally cannot have his PC let go and fall if the DM declares the climb an automatic success. Nor could he jump off of the wall if the DM declared the jump an auto failure.
Clearly those things cannot be true, so the player must be able to trump DM declarations of auto success or failure at least some of the time.
Sure. I want to climb the wall. Most of the way up I can now see that on the other side is a veritable army of enemy archers. Now I want to get down in a hurry by letting go and falling. Except the DM has declared auto success. Am I forced to complete the climb and be on top of the wall? I don't think so.