Audio problem: When Oliver is fighting the guys in the hallway, he shoots the second to last guy. But when he starts to draw an instant beforehand, you can see that his right hand, with which he holds the arrow, is nowhere near the quiver when the sound of the drawn arrow begins.
Arrow (2012)
1 audio problem in The Undertaking - chronological order
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: Although Laurel is a couple of years older than Sara, Katie Cassidy is only about a month older than Caity Lotz.