I dunno, for me, a misalignment or even some broken optional action somewhere that you just didn't think to check while testing are one thing, but major sky tiling issues in open areas are such an intergral part of geometry that you might as well have left a required key out of the map, or forgot to make an exit anywhere, and also said "hey we can't all do perfect on the first try". I suppose it's just because I always look around every new area I've made immidiately, being especially careful when sky's involved, because there are all sorts of issues with Doom's sky that I can't allow to slip by, such as cutoffs, HOMs or tiling. Seeing sky tiling glaringly there like that, to me, almost gives an impression that you didn't run through your map ingame to see how it turned out even once (which, I know, can't be true, but still), because I can't really imagine missing sky tiling that obvious - if you look over the railings even briefly, especially in the end area, it's very hard to not notice.
This is a highly problematic response, as dangerously close to "why didn't you produce perfect results the first time?"
But oh well! At least you're fixing it :P
I hope you appreciate all the tedious work I went through to make the automaps of my maps look as clean as possible without any line that isn't informative of the playing layout (such as ceiling height variations ans whatnot)!
I'm pretty much an "automap specialist", I always catch automap bugs and report them. I think I annoy the hell out of some people doing it, but yeah, the look of the automap is important too.