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
Starring: Katie Cassidy, David Ramsey, Stephen Amell, Colin Donnell, Emily Bett Rickards
The Magician - S3-E4
Revealing mistake: When Oliver meets Thea at the Grind and Jot Cafe (Which looks unsurprisingly like CC Jitters) you can see the cups on the table are from CC Jitters. (00:20:50)
Darkness on the Edge of Town - S1-E22
Oliver Queen: Roy! We haven't met. I'm Thea's disapproving older brother.
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.