The storyline is based on the light novel series "Sword Art Online" originally intended as an entry into a short story competition. The original novel was twice as long as allowed for the contest but Reki couldn't bear taking anything out of the story leading to it being self published.
In the English Dub of Season 2 Ep. 19, Asuna, while Glaring at someone, actually says the word "Staaaare." This was improved by her voice actress, Cherami Leigh, who didn't even know it was left in until it aired.
Episode 5-6 were after mothers rosario in the light novels however they were put in the first season while mothers rosario arc was put in the second season
In the first episode. As Kirito is reading his magazine. You see a small glimpse of his computer screen with the news report of SAO and Nerve gear along with social media. In the report if you look closely you'll see Klein, in the bottom left corner with his friends holding up their copies of the game.