Doomsday Engine

Does this occur with all of such walkable bridges or is it specific to certain maps/mapping constructs?

I recall this logic was changed a few versions back following a report of players not spawning at map start due to collision with such bridges. Perhaps a new bug was introduced in the process.

You are right that it doesn't seem quite as simple as I mentioned above. I have found that I can walk on star bridges in some circumstances.

Using the preplaced star bridge in the north quadrant of Menelkir's Tomb map (visit 28) seems to trigger the bug every single time though; on that one, I can't see to get beyond the second star bridge mobj without falling through.

Further experimenting seems to reveal it's consistent and very specific star bridge mobjs. Seems to be ones that are next to ledges specifically (i.e a long star bridge crossing a canyon; it will be the first couple at either end that are likely to have this issue).

This doesn't appear to be limited to the star bridges. A quick check in Hexen shows that jumping on pretty much any mobj can lead to the player getting stuck depending on the velocity at which the player is falling. For example, jumping out of the bell tower in Winnowing Hall and onto a tree the player often falls straight through and becomes stuck inside it.

That sounds like number 2 that I posted in this old report:

http://tracker.skyjake.fi/issues/1129

Though I didn't consider HeXen in that report (in the original report) as I thought Dday had two implementations of Z clipping; that HeXen used Vanilla HeXen's and Doom and Heretic were using a Deng Team made implementation. I was testing Deng Team's implementation in Doom and Heretic.

Sorry, I made a few typos and mistakes in my recent post. It should say: "Lower the pillars with the cave key and enter the new area, using the teleporter." In the next area, that's where the star bridge is.