Bonus Material: Besides the story notes, the PDF-Version sports scene pictures and actual bonus material, becoming this.
There even is a bonus story, called Impact Day. It can be read here and also is available in the PDF-version.
Limited Special Collector's Ultimate Edition: There also is a PDF-version of the story, which not only has superior formatting, but also contains countless scene pictures (either staight from the source, manipulated or totally original). It however is only avaiable by request.
Schedule Slip: That was very common when the story was written. Up until Chapter 17, the update schedule was brisk - though quality suffered from it. It slowed down considerably after that chapter.
Shown Their Work: For Misato's and Kaji's wedding, the author worked really hard to get as much information possible about Shinto weddings, so that there wouldn't be cultural ignorance at work. The amount of work does show.
Throw It In!: Happened a lot due to the author only having the basic story outlined, filling it with the details on the go. At the beginning, the author not even had a plan how to finish it, though that changed after a few chapters.
Hearing the thoughts of the Angels was a spur of the moment decision. There was no plan what it would result in.
The story actually started like that. Only later, actual planning was done.
Updated Re-release: The REVIVAL-Edition is a massive update of the whole story. It did start with a desire to fix the wonky grammar and escalated into rewriting a big part of the story, due to how unhappy the author was with the rush job. The end result is a much better read that also is much longer than the original story.