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 used to date 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.
Character mistake: When Oliver is brought into Fires' tent, he gets pushed into a pole by one of the guards. Take a close look at his handcuffs. They are so loose, he could easily escape from that. Not only that, but no guard who had any training would ever put handcuffs so loosely on someone they would treat as "hostile".
Factual error: Felicity apparently extracts DNA from the tooth of one of Damien Darhk's men. She then realises that the DNA lacks "half of the genetic markers" by looking at an optic microscope. Skipping the fact that this sentence makes absolutely no sense whatsoever, you certainly cannot analyse genetic markers by simply looking a tooth with an optic microscope.
Character mistake: At one point Felicity says, "There's at least a teraflop of data to go through." Felicity is a computer genius, so she should know that a teraflop is a measurement of processing power, not data storage. She should have said "terabyte."
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: During the hallway gauntlet, Oliver takes a guy out and ends up on his left knee. He then starts to turn right and draws an arrow, which doesn't make any sense because he holds his bow with his left hand and can't get a good shot from that position. What's more, in the following shot, he is turning to his left.
Factual error: Felicity tracks the abducted team members to latitude and longitude values that are negative, which she and others present regard as impossible results until they realise the captives are in outer space. In actuality, south latitudes are conventionally represented as negative, as are west longitudes; and negative values would not convey information about an outer space location, which would instead be referenced with celestial coordinates.
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.