Alright, so here's a weird one.

Basically, there's a point (really, a whole vertical plane) in my map that sends you into the void--whether you're "in" the map or not.

It's kind of hard to describe so I uploaded a video:

https://drive.google.com/open?id=0B5ubevjSCiEYZFZlX2VUQzZnN1E

As you can see, if you move away from the blue bridge structure towards the blackness, you pass into the grey void without actually reaching the end of the box--and as I demonstrate later, this happens outside of the map bounds as well. Actually, you're technically taken to the *other* side of the map--as if you'd gone so far in one direction that you looped around to the other (like what happens when you fall through a hole in an unsealed map).

I *don't* think it's because I exceeded normal map size limits (that's what those colored lines are supposed to represent in TrenchBroom, right?). I've already ruled out holes, at least those that BSP can detect, having tracked them all down with pointfiles and removed them. Any ideas? 

That really does look like hitting up against the hard limits of map size, actually. I'm not aware of any coloured lines in TrenchBroom that represent the bounds of the map, either, just the set that extends out from the origin along the various axes. My advice would be to spawn an entity out there and check its "origin" key values, or use the viewpos command to check the player's own coordinates.

If you're not actually outside the map bounds, i'm totally stumped. 

I had this issue when I exceeded the 4000 from origin limit 

That might be it, although those flames are just past 4000 units from origin (e.g. one of them is at -224 4048 460 according to TB).

The colored lines are visible here:
https://drive.google.com/open?id=0B5ubevjSCiEYT1JfRF9SODBjZG8
I thought I read somewhere they extend as far as some (extended) limit but I guess they're just the axes. To be honest, I thought the map size was a bit bigger.

There are bigger maps though, like ORL's Zeangala... how does that work? 

cause it's a super secret area that I doubt many of the ten whole people who will play the map will get to, and they'd fall to their deaths if they jumped from there anyway. :P 

You could Select ALL and drift the entire map's contents a bit away from the boundary limit. 

4096 is the actual limit, not 4000. 

I have been reading ad docs, especially trying to figure out how to make door open in a way.. I can specify origin where door rotates 90 degrees, and probably will just stay open after triggering, because if it spins back after some time "wait", it should notice if it touches players or enemies so it goes back to open..

Noir thmemed map, wooden doors might look stupid if they just moves inside walls. 

Sorry typos.. anyway there is multiple options..
'func_rotate_door' + 'info_rotate' might be things I should use? 

You can use sv_protocol 999 console command in the latest Quakespasm, 0.92.0. That expands the quake boundary limit from +-4096 to +-32678. There's no special compiling tools or commands to use either, just set sv_protocol to 999, restart the map, and you should be golden.