Not exactly a bug either, but ...

[Sclerocephalus]

At Red Rocket station, I built a large shack on the roof and stairways on both sides of the building to get up and down. For some reason though, my settlers appeared to have problems with the stairs. Every once in a while, I heard someone crying out loud and sometimes I could watch settlers falling down from above. So I started to investigate why this happens.

 

You can see here why:

 

http://www.afkmods.com/index.php?/gallery/image/4476-redrocketnonsense/

 

Some joker added a jump down ledge marker to the edge of that blue wooden plank [NB: can't get a refID, the console always tells me something about this being a 'non-reference AV object']. In fact, the shortest way to get down from the roof is by jumping. Settlers are only using the stairs to go up.