When The Canary saves a girl from the fire, a blanket is covering the girl in every other scene.
Malcolm's arrows disappear right before the Mirakiru soldier tells him he's out.
(at around 3 mins) Quentin Lance looks at a door labeled "Weapons and Ordinance Storage." "Ordinance" means a law, regulation, order, etc. The word that should be on the sign is "Ordnance," which means military weaponry, armor, etc.
When Oliver climbs out of the rubble from when he blew up the ceiling, the rocks and chunks of concrete are clearly made of styrofoam, as many of them slide and roll off one another without Oliver touching them.
Diggle's stunt double is visible during his fight with Isabel.