I was playing CB and after I left 's chamber, I went into a T-Shaped Hallway. One door led to the First Room, and the other led me to a void. When I fell in, I fell into the Maintenance Tunnels.

I get the T-shaped hallways before the first room sometimes. One path almost always leads to a dead end and the other leads to the 'first' room. Never seen a void before.

I even removed my mods, I can't fix it! This is really odd, I reinstalled and everything!

Sounds like a bad bad map seed...

Yeah this has happened to me on a few occasions... It's a pretty rare bug though and you can usually get around it, but it is annoying.

This seems to happen in some map seeds, but for me, they appear before the heavy containment zone. I only fell in the Maintenance Tunnels once, as I mostly noclip into the nearest hallway, which is normally towards the end of the heavy containment zone.

I've had this problem to! Every time I encounter it, it always has the event where the scientist comes from the ceiling (You know by this dude --> ). They only way I get around this is by finding the first room. I do this By going in a random direction and then if its the wrong way I go the other way. (If you get what I mean)

Edit: I understand it more now, its kind of a tweak, one leads to a end room and there was a new event there (It scared me, I didn't know that would happen, but hey, the janitors running animation is funny though.) and the other leads to the closets room, however there is a bug where it will be a lock room (with the gas)
and if you are lucky, you won't have SCP-173 in there!

This is clearly not a bug to me. It's more-so a little tweak that Regalis made. As for the other direction, I get an endroom instead of the void.