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: 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.
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".
Continuity mistake: In the previous episode Ra's al Ghul stabs Oliver through his chest, just below his left pec. When he pulls out the sword, a wound is visible. (However, seconds later, the wound is absent - different mistake). In this episode he is shown with a wound much lower (in his lower abdomen) and also later, with a bandage around his waist instead of his chest. (00:09:30 - 00:41:30)
Factual error: When Oliver is hunting Prometheus, they come face to face. Oliver fires an arrow, which is tracked in slow motion before being split by one of Prometheus' throwing stars. Problem is that Oliver's arrow is, well, flying straight as an arrow. But in reality, when you film an arrow in slow motion, they flex and wobble, and definitely wouldn't fly like the one shown on screen.
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. (00:11:05)
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. (00:36:50)
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)
Factual error: The highest-ranking military officer in the DIA (Defense Intelligence Agency) is the director, which is a three-star position. John's stepfather Roy Stewart would not be a part of the organization, let alone answering to superiors in the organization if he was a four-star general.
Visible crew/equipment: During a flashback with Oliver and Maseo, they are on a rooftop trying to break in to the building next door by means of the pulley system on the bow. Maseo is shown lining up a shot with his sniper and in the background one can see a crew member's face looking over his shoulder. (00:13:30)
Deliberate mistake: During the big battle, Arrow is knocked down and when he opens his eyes, the reflection in his eyes is tilted over to give the sense he's seeing things sideways. But the reflection should still have shown the proper orientation no matter what orientation his eye was at. Think of looking into a mirror and then rotating the mirror to the side, the reflection wouldn't rotate.