Deliberate mistake: The "chest compressions" Diggle performs on Oliver at the beginning of this episode are laughable. He's barely applying any pressure at all, and his arms are almost straight out in front of him instead of straight down. I would have called this a character mistake since Diggle really ought to know better, but this is more of a safety issue than an oversight; real chest compressions tend to break ribs.
Arrow (2012)
1 deliberate mistake in season 2
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)
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.