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.
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.
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.
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.
Factual error: When The Hood busts into the underground casino, the throws a guy through a glass table, and it breaks into large shards. He then proceeds to grab a handful of safety glass, so it's not the same glass anymore, and he throws it into the face of a guard, and it hurts him badly. Safety glass is designed precisely to prevent that. It is impossible for the pieces to get lodged in the guys face, no matter how hard The Hood throws them.