Hey TWHL, long time no see. I've got a problem I've never personally encountered before and haven't been able to dig up any info on through Google. Basically, after compiling my map I've found two instances where there are invisible walls that can be passed through from one side but not the other.
The first I've encountered is through a doorway, which can be passed through normally going the direction you're supposed to be travelling, but if you try to backtrack you'll be blocked. Even more oddly is that sometimes it seems like you can jump over or crouch under this wall to bypass it.
The second appeared in the middle of a random hallway, but also has disappeared completely after recompiling the map with no changes made (the first one has remained despite multiple compiles).
Obviously I didn't construct anything of the sort in Hammer, and I don't think I'm seeing any errors in the log either.
If you're interested, here's a link to the map files. Maybe someone can see something I'm missing, it has been 4 years since I made a map after all.
You'll find the invisible wall right in the first doorway if you try to backtrack once walking through it.
https://www.dropbox.com/sh/mlcl4hef5yzwg5v/AAA3Sxt9SEn1AzNpPzp-n7Vva?dl=0