Art Shift: The fan-fiction initially follows several characters' viewpoint in each chapter. The author later decides to cut down on them for the benefit of producing each chapter faster without spending too much time detailing several characters' actions at once.
Hilarious in Hindsight: Long after the story began, the video gameSword Art Online: Lost Song introduces Argo as a Cait Sith. (Though this isn't the only fanfic where she's a Cait Sith...)
The fanfic author was amused to learn that Philia - the Canon Foreigner from the video game Hollow Fragment- was going to be a Spriggan in Lost Soul. That's because he was going to include Philia in the fanfic...as a Spriggan.
His choice to set PoH as a Spriggan falls into this when you consider that guy's real name from the source material. His real name is Vassago Casals, and Vassago can refer to a Goetic demon who was apparently good at revealing hidden treasures, fitting in with the in-game Spriggan lore. Again, it falls under this trope because the author wasn't aware of this connection.