So, take a look at this picture. https://www.dropbox.com/s/qesx97ld8x5nym8/THIS_2.png
Where that red box is, there's supposed to be a door.
But instead, the walls surrounding it seem to stretch and cover it.
The thing is, the part covering the door is nonsolid, meaning that you can see it, but it's walk-through.
I have no clue what causes this, and I would really appreciate some help on how to stop this.

Make sure that the part of the wall you are referring to is not a microbrush. This kind of thing has happened to me as well, for that reason.

The part of the wall in the red box isn't even supposed to exist, period.

Yes, I know. Its probably an extruded vertice from a microbrush. Try deleting that segment around the door and recreating it.

Have you been doing any vertex editing? I recommend using the 'check for problems' menu option frequently, it tells you whether you have a bad brush.