Deliberate mistake: Whenever Deathstroke is on screen, he has three grenades for a launcher strapped to his chest. But on closer inspection, you can see these grenades are actually BB-gun grenades: You can tell by the very distinctive holes at the tip. They did this because launched grenades are A) very expensive and B) not very safe to walk around with on set. A few of the most noticeable times are when Yao Fei is fighting him when he is freeing Oliver in episode 5 and when Oliver is talking to him in episode 14.
Arrow (2012)
1 deliberate mistake in Season 1 generally
Starring: Katie Cassidy, David Ramsey, Stephen Amell, Colin Donnell, Emily Bett Rickards
Al Sah-Him - S3-E21
Character mistake: Diggle turns to Nyssa and says "I think what she's trying to say is, Oliver would never do anything to hurt you, Felicity," incorrectly addressing Nyssa as "Felicity." (00:14:50)
John Diggle: The knife.
Oliver Queen: Right. The knife. I got lucky.
John Diggle: That was a kitchen knife. It wasn't even weighted properly and yet you threw it with accuracy across a ten foot room.
Oliver Queen: Exactly. I got lucky.
Trivia: When Barry and Oliver are fighting on the street, a police officer asks for backup and mentions Burton and Nolan street, as reference to Christopher Nolan and Tim Burton.
More questions & answers from Arrow
Suggested correction: When Diggle is walking away from Felicity, he is still talking to her but it looks like he is talking to Nyssa.
Stephen Edmonds 1
People have already tried to correct this mistake. It is valid. In the scenes prior, Nyssa says Oliver will do what it takes to kill her and Oliver says "tonight, Nyssa Al Ghul faces justice." Nyssa is there to bring the team the news that Oliver is going to try to kill her. Felicity says "Oliver wouldn't", meaning Oliver wouldn't try to kill Nyssa. Diggle says "I think what she's [she being Felicity] trying to say to say is that Oliver would never do anything to hurt you" [you being Nyssa whom Oliver is going to try to kill]. He is meant to say "Nyssa." Otherwise the entire line means Nyssa or Laurel are there to convince Felicity that Oliver would never do anything to hurt Felicity.
Bishop73