I’ve got a save with this bug. I started this character in experimental 4001 or so, I’m playing in 4008 now. Load up Cannon Falls, only one character. He is standing on a downward stairway into a lab, Press < and you will get a dialogue about climbing up, apparently onto the nearby wall. The oddness really kicks in if you walk around a bit, try going east a couple squares. I fell down 2 levels to the 1st underground level of the lab. I’m not sure this is even a problem with the climbing code, rather a problem with there being no ceiling to keep you down and no floor once you get on top of the bunker. The falling through the ground to the basement thing is bad thing though.
Upon loading there will be a lot of invalid terrain errors, just space through. I’m using a lot of mods
https://drive.google.com/file/d/0BymjOrrNny5BdlJORnFYZFktREk/view?usp=sharing
edit: I did a bit of testing. If you go into the room north of the stairs you start on and stand in one of the corners by the counter, you are also able to climb on the roof of the bunker, which takes you to the blank sky land, in which walking around will lead to falling through the ground level to the underground lab below. It seems like this is a problem with being able to get to a place with no floor that people only really notice when on some stairs.
edit2: This can be reproduced fairly easily without the use of stairs. Start a default character (evac shelter start) in experimental 4008. Give yourself enough strength to move a locker, I had 12 when I did this. Walk to the nw corner of the shelter and drag the locker out of the corner. Stand in the spot the locker was and press <. Pick 1 to climb northwest. This will bring you to the all blue ‘sky world’ which is simply what the world looks like with no terrain. Walking northwest will drop you to the ground outside the shelter, walking southeast will drop you back where you started. You can south or east the length of the shelter even though you can’t see what you’re walking on. I wasn’t able to reproduce falling through the floor to the basement though. That’s in the savefile.