a concept that's fun to think about is an "unplace" (a term i am making up).
in most "worlds", things exist "off the map", so to speak. there might not be anything there, but it exists. if you go off the edge, you might end up nowhere, or an empty room, sure. but you can go there.
like in real life, you can technically go anywhere. everywhere exists; even if it's solid rock or empty sky, you can, in theory, move to that position and exist there.
in this world, that isn't the case. empty spots on this map do not exist, in any shape or form. you cannot go there. under no circumstances can you "boundary break" and visit one of the empty spots on this map.
(of course, this whole map is a fabrication; it does not exist in the game world.)
consider link's awakening, above; this is eagle's tower, the seventh dungeon, from an "internal" perspective (src). the map is stored as an 8×8-byte array of room IDs. the repeated room you see everywhere is room 00; on the first underworld, it's an empty room, but in the second underworld, it ended up being a copy of the top-left corner of 1F.
what you see here is actually how this game works. if you are on one screen, and walk off of it, whether by cheating or otherwise, you will move... one screen to the right. the "empty" screens are still there, even if they're not intended to be visited. what you see is what you get.
in the case of mighty bomb jack... that isn't how it works. the "maps" people, including myself, have made, aren't actually representative of the world. much like how a 2d projection of the globe has to take some affordances to account for the loss of its 3dness, mighty bomb jack's map is a transformation of how it works into a "normal" representation.
take the labyrinth on the right. the middle, leftmost room, D1, is adjacent to D8 (right). but you cannot get to D8 from D1. i don't just mean that there's a wall there: i mean even if you were to insert a door into that wall, and go through it, you would end up in a void1, room 00. and from there, you're literally off the map -- room 00 isn't "on here", because it isn't anywhere.
to put it another way, when you're standing in D8, the only other rooms that exist for you are DE and D9. "the world" doesn't exist: the world, quite literally, revolves around you.
it's just weird to think about, when the game world otherwise fits together 1:1. it feels like it should work like any other map. it just doesn't. and it's fun to pontificate about the deeper meanings of that.
(1. you would have to alter the room-neighbors table to make it work, which is kind of the crux of this issue: it's not a coordinate plane.)
Neighbor-table space is common in parser interactive fiction - Colossal Cave used it for tricks such as one-way passages, , corridors that arrive at a room in a different direction they left the previous room from, and multiple directions leading to the same room.
An early notable use is in Atari Adventure, where mazes that only occupy five screens at most are turned into a tangled mess of corridors that's impossible to cleanly render in 2D space.