In the promotional picture for the series it shows a shirtless Oliver. The scars from his time on the island are visible, but there is no tattoo. During the series Oliver has a tattoo on his chest, a tattoo that plays an important role in at least one episode (1x03 "Lone Gunmen") and has been visible in every other episode.
In two second-season episodes, chest compressions to restart Oliver's and Roy's hearts were done incorrectly. They are pushing down on the person's actual chest, rather than two finger widths above the sternum. That would damage the ribs, lungs and other vitals.