I present you, a mid level thief, demonstrating her Climb Walls Extraordinary Ability.
I rule failed climb walls as "No convenient crack, etc. was found" rather than falling. Damage is too severe penalty for attempting something that is a big reason to even bring thief along. I want thieves to be utilizing their abilities. I want parties to be thinking of and executing interesting solutions to obstacles using Climb Walls.
If thief gets attacked mid climb or other "bad thing" happens, I'd probably require an extra Climb Walls check to "hold on". And a Save vs Petrification (my go to "react to avoid bad thing" save based on some early published modules) for half damage.
Also why I don't make it insanely hard (as written (esp in AD&D) or interpreted by many) to set up a back stab. If thief successfully "sneaks*" prior to combat, they get back stab on first baddy passing them by. During combat if they spend round sneaking (and are successful) the next round they get to backstab **any** enemy. Party can choose to take out leader or wizard. But, only by investing 2 rounds (with possible failure) and probably leaving thief vulnerable "behind enemy lines". Another tool / decision point for players to exploit.
* my house rules combines move silently / hide in shadows into one skill sneak. Sneak is the thematic thing going on. The implementation details (hiding in shadows, using distractions, cat stepping) are beneath the abstraction level of D&D.
Original Reddit thread https://www.reddit.com/r/whatcouldgoright/comments/lombqq/i_mean_the_stairs_were_right_there/