The lone sidedef

What is the purpose of the sidedef that is sitting outside the map on level 30?
I've noticed this ever since I sat outside the map for a half hour waiting for it to get populated so many years ago... baffles me.

Maybe thats what made those freaky errors I saw on the walls before. If I remember correctly, there would sometimes appear to be a door or a gap in the wall sometimes if you were at the right angle. I used to sit in that level all the time and blast demons before my pirated version of Doom 2 f***ed up...hmmm maybe I should have put that in my confessions too (see above post).

Just a mistake. There are several levels with mistakes. As I recall, Map14 is one that has quite a few.

PS: That's a linedef with a sidedef. Sidedefs (and sectors) are never visible without a linedef:))