Line 0 has no first side?

First it was one map, now it is 2 of my maps. When the level starts, the Zandronum error window pops up, and the console says:

ACS scripts unloaded.
Line 0 has no first side.
Removing 0-length line 3

So, why is this happening?

Are you using GZDoom Builder? I remember bugfixing a map for someone else that used it and found a corridor with no variables on any of the one-sided walls. Look for the line in question and make it impassable.

Obsidian said:

Are you using GZDoom Builder? I remember bugfixing a map for someone else that used it and found a corridor with no variables on any of the one-sided walls. Look for the line in question and make it impassable.

I am using Doom Builder 2. I went to Linedef 0 and "impassable" was already checked.

EDIT: It works when I test it in Doom Builder 2, but when I put it in my PK3, it shows the error

Upload the map for us to look at, please.

Doomguy94 said:

EDIT: It works when I test it in Doom Builder 2, but when I put it in my PK3, it shows the error

This is possibly because your DB2's testing port is (G)ZDoom, but otherwise you play in Zandronum. Zandronum is generally more touchy about geometry errors, and more prone to refuse to even start the map if they appear.

scifista42 said:

Upload the map for us to look at, please.
This is possibly because your DB2's testing port is (G)ZDoom, but otherwise you play in Zandronum. Zandronum is generally more touchy about geometry errors, and more prone to refuse to even start the map if they appear.

Ok, here is my map...

https://www.mediafire.com/?4fs7a58g8mp6nb3

I don't get any errors on that map, neither in editor, nor in Zandronum (version 2.0), nor in ZDoom (version 2.7.1).

scifista42 said:

I don't get any errors on that map, neither in editor, nor in Zandronum (version 2.0), nor in ZDoom (version 2.7.1).

Maybe if I upload my pk3...

Maybe your pk3 contains an older glitched version of your map, while this one is already fixed. Try moving this very map that you've posted here into your pk3. If it doesn't help, and you keep getting errors with the pk3 but not the map's wad, post the pk3.

scifista42 said:

Maybe your pk3 contains an older glitched version of your map, while this one is already fixed. Try moving this very map that you've posted here into your pk3. If it doesn't help, and you keep getting errors with the pk3 but not the map's wad, post the pk3.

Well, I removed the lines in question (linedef 0 and linedef 3), but it still crashes and all the console says now is

ACS scripts unloaded.

EDIT: Problem solved(?) I just copied and pasted the level onto a new Doom Builder file.

"ACS scripts unloaded" alone (without further error messages) can display when your map contains a BEHAVIOUR lump but there is no actual ACS code, because you didn't write any. In that case, the message (often) doesn't indicate any real error at all and can be ignored, as far as I know.