2 hypothetical situations:
What happens if a minehole is built where the ladder placement would be exactly on top of an unliftable object?
ie: cave level minehole is built directly above a mine support on level 2.
What happens if a minehole is built where there is something occupying the space where the 7x7 space would be?
ie: a cave level minehole is built above a level 2 minehole, not necessarily where the ladder would be but still occupying the space.