Restoration - S4-E3
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)
Suggested correction: No science behind this statement.
What is it supposed to mean?
Blood Debts - S4-E10
Continuity mistake: When Conklin is whipping Oliver during an island flashback, the wounds from the whipping are visible, but after Oliver threatens to shoot himself unless Taiana remains unharmed he hands the gun to Reiter, and we get a good look at Oliver's back, wound-free.
Continuity mistake: Oliver chops off Malcolm's hand to end their duel. When Oliver hands the ring over to Nyssa you can see Malcolm lying in the background where he has been cut down with two hands intact, and the cut off hand nowhere in sight.
Visible crew/equipment: While the camera is moving from Black Canary and Speedy to Spartan you can see on the ground the shadow of the camera man walking. (00:33:45)
