A flashback in "Heir to the Demon" reveals that Mrs Lance was a Professor. Alex Kingston has played a Professor before.
Creator Backlash: A mild example, compared to others — Stephen Amell has quite happily gone on record with his displeasure over the marketing for Season Two...namely that the posters highlight the ensemble cast, but leave out Paul Blackthorne and Susanna Thompson.
A number of people suggested that due to her brown hair not being a trait inherited from two blonde parents, Thea wasn't Robert and Moira Queen's daughter. It was revealed in "State v. Queen" that Thea was conceived after an affair between Moira and Malcolm Merlyn.
Malcolm is Back from the Dead. Okay, technically he never died in the first place, but the point stands.
As soon as Sara asked Oliver "What happened to Slade?" without mentioning Shado, people correctly assumed she was killed.
Slade's return. Given the character's popularity, most fans knew he wouldn't be killed like he was. Also, since the character is known for being a villain, as soon as they mentioned what Ivo was looking for and what Brother Blood was doing, many worked out he would be used on Slade and that he was the Big Bad well ahead of the reveal.
Brady reappears five years after a huge shipwreck that claimed the life of his father and Olivia among others, and becomes a bow-wielding vigilante. He's still hung on his girlfriend Amanda, who's Olivia's sister and is mad at him for multiple reasons, as is her father, Dr. Lawson. His sister is also Marissa's.
Mr. Blank was originally supposed to be the DC Comics villain Onomatopoeia. However, as Onomatopoeia only speaks in, well, onomatopoeia, they couldn't make it work without him coming across as mentally retarded (since sound effects sound far different in real life compared to comic books).
The twentieth episode of Season Two was originally going to be a Back Door Pilot for a potential spin-off focusing on The Flash. Based on both the strength of Grant Gustin's portrayal of Barry Allen in "The Scientist" and "Three Ghosts" and the fact that using the twentieth episode as a Back Door Pilot would interrupt the show's momentum as it built up to the season finale, the decision was made to make it a standalone pilot.