Continuity mistake: In season one, Sara is a brunette in Oliver's flashbacks. In this season's flashbacks, she's a blonde (most noticeably the ones aboard the Queen's Gambit).
Revealing mistake: When Oliver is threatening Mathis' lawyer, he shoots him in the shoulder. When Oliver grabs the arrow and pushes it through the shoulder, there is a significant difference between the angles. Where the arrow is coming out, it is practically horizontal, but in the chest, it is at a thirty degree angle.
Factual error: There is no way Laurel Lance would have been allowed on the prosecution team for Moira Queen's murder trial. Due to the facts that she both dated and as a lawyer represented the defendant's son, the defendant's daughter used to intern for her, and her boyfriend was one of the people killed in the Glades, it would have taken all of two seconds for the court to have her removed if she didn't willingly recuse herself, since her presence would be a major conflict of interest.
Three Ghosts - S2-E9
Factual error: When Cyrus punches through the windshield of the truck, it shatters into hundreds of little pieces, like safety glass. Windshields are not made out of safety glass.
Revealing mistake: When Oliver breaks into the building and draws his bow, there is a laser pointed at the tip of his arrow. He is told that if he moves a muscle and removes the tip of the arrow out of the of the laser that he will die. But shortly afterwards, just before Oliver looks over at the TV, you can see the arrow tip move out of the way of the laser and see the laser reach the other side of the wall for a split second. (00:29:10 - 00:29:50)
Factual error: In this episode Barry uses rat poison to save Oliver, from a poison that coagulated blood. Though rat poison contains anticoagulants like Warfarin, this is medically wrong because, It takes at least 1-3 days for Warfarin to be effective and it may even cause excess of coagulation during this time. It could never have saved Oliver instantaneously. What Oliver needed was heparin. So Barry in real world instead of curing Oliver might just have made him much worse.
Continuity mistake: When Oliver tries on his mask for the first time, he is not wearing eye make-up. When the mask is on in the next shot, he suddenly is.
Deliberate mistake: The "chest compressions" Diggle performs on Oliver at the beginning of this episode are laughable. He's barely applying any pressure at all, and his arms are almost straight out in front of him instead of straight down. I would have called this a character mistake since Diggle really ought to know better, but this is more of a safety issue than an oversight; real chest compressions tend to break ribs.
Continuity mistake: When Bronze Tiger is stealing the prototype earthquake device, when the truck is leaving Merlin's garage, the street shown has street lights and a factory building, not a country estate driveway. (00:17:10)
Visible crew/equipment: When Slade brings the drink in the forest (referring to Billy Wintergreen), you can see shadows of people walking upstairs in the background behind a big white curtain. (00:07:30)
Suggested correction: It's not a crew member, it's wind.
The Man Under the Hood - S2-E19
Continuity mistake: In the first shot of Thea running out of Slade's car, she closes the door. In the second shot, you can see the door of the car open. (00:02:05)
