Can you climb walls? I can. Can you scale sheer surfaces without rope and gear? Err...
When I run D&D (and clones thereof) I always try to remind myself that the Thief 'skill' percentages are so low because they are not, in fact, skills. Anyone can climb or hide, and any competent locksmith can open a normal lock. The *proper* (ahem!) interpretation of Thief 'skills' is that they are preternatural abilities, beyond that possessed by ordinary humans (and beyond that of even the extraordinary people with classes and levels with whom they adventure).
This, though, takes some overlooking of what the rulebooks actually say. The rulesbooks often describe most of these abilities in utterly mundane ways, and in the case of the Thief's 'climb' skill/ability I had been dimly aware that, at some point, even the name was mundan-ised, from a version that refers to scaling 'sheer' surfaces to one that merely refers to climbing walls. Bah! I thought I'd check through the versions of TSR D&D that I own to see what each edition has to say.
Moldvay Basic. Promisingly, the Thieves' Abilities table has 'Climb Sheer Surfaces'. Unfortunately, the accompanying text has the ability listed as 'Climb Steep Surfaces'. The description of the ability does, however, lack the kind of specificity that prevents the ability being interpreted in a preternatural way. (Moldvay B88)
Mentzer Basic (and the Rules Cyclopedia) boils down the ability into the boring and not at all preternatural 'Climb Walls', and the text is unhelpfully specific, saying that this ability "applies to any steep surfaces, such as sheer cliffs, walls, and so forth." (Mentzer B44)
Note: Both Mentzer and Moldvay promisingly describe what we commonly call Thief 'skills' as 'special abilities', which is at least suggestive of these chances being something different, over and above what a mundane person ought be able to do.
AD&D1e has the ability - in the 'Thief Function' (ack!) - table as 'Climb Walls'. The description is utterly mundane. Also, in an amusing Gygaxianism, at high levels, this 'function' is not adjudicated by way of a normal percentile roll. After 10th level, the base chance to 'climb walls' increases beyond 99% by a tenth of a percent each level. Talk about marginal gains! The player (or DM) therefore needs to roll a D100.0 - three ten sided dice. My goodness, AD&D1e is one of the least lovely editions of the game. (PHB1e 28)
AD&D2e might have 'reduced' the special abilities to 'skills'. It might have 'Climb Walls' rather than Climb Sheer Surfaces. But by Crom it gets the description right! "Although everyone can climb rocky cliffs and steep slopes, the thief is far superior to others in this ability. Not only does he have a better climbing percentage than other characters, he can also climb most surfaces without tools, ropes, or devices. Only the thief can climb smooth and very smooth surfaces without climbing gear."(PHB2e 40)
I had expected to find that earlier editions of the game were open to a more preternatural interpretation of the Thief's climbing ability, and had expected that, at some point (under the influence of skill-based RPGs) that this would become mundan-ised as a general climbing ability, effectively disallowing other characters from climbing. What I didn't expect to find was that AD&D2e (which I already have quite a soft spot for) is the only edition of those that in which the Thief Special Ability/Function/Skill is expressly something above and beyond that possible for other characters.