A stairway to an upper floor in the safehouse I was using got trashed by zombies, so I bashed it out and replaced it with a stepladder.
However, after saving and loading back into the game, the space above the stepladder has transformed inexplicably into a ‘flat roof’ tile, making it inaccessible and removing the only way down in this house. 
Anyway, climbed down the side of the building and removed the ladder from below to set it up elsewhere. The weird new ceiling/floor tile persisted.