Well, this is interesting...
In the upper left corner of my base (14 left, 28 up, from the center of the portal) is the tile shown in the attached image. Before I penetrated the chlorine bubble above it it appeared as granite, but I could not place a ladder over it's location: the ladder was red.
Now that it's been excavated (I think when the airlock below it was installed?), I can see what it is: a "Steel door <DO NOT TRANSLATE>". I can't build, dig or do anything in that tile. Judging by it's stats, it actually is made of steel.
My duplicants can jump through it so it's not keeping anyone from doing their job, but I could imagine an obstacle like this causing problems.
It is visible in the uploaded save file.