There was I, all bold and marvelous through the Lowerdark, hunting for those areas Howland keeps trolling us that we've never visited.
That's when I've bumped in at least three transitions that refused to allow me see all those marvelous and terrifying things that are kept hidden from the common player.
- Upstairs near the red cross (one of the first maps)
- The one downstairs from the Blasted Pits (sorry, I'm not quite sure of the map)
- Sand Cavern - The Devoid - south (upstairs, this one is better detailed, no?)
The places you can't go to are likely areas where there is a Quest!
Pretty sure they're all placeholders for eventual zones.
I used to wander the Lowerdark a fair bit, and they are indeed broken.
Pigadig is correct.
I had asked Howland about this on my first real exploration character. He confirmed that the transitions connect to nowhere. They might later be utilized as part of a Lowerdark expansion or dungeon master's quest, but there is no marvelously terrifying content just yet.
I was told by a DM that at least one of them were bugged the first time I noticed this issue. So... *shrugs*
... oh, and yeah. Howland has sad that there are areas of the Lowerdark still unexplored.