Continuity mistake: Gar enters a room and leaves the door open. Throughout the whole mirror scene the door is also open both when the camera looks at him and when the camera points at the mirror. However, when he is about to leave the room he needs to open the door as it has been closed inbetween us looking at Gar in the mirror and looking at him normally. (00:17:50 - 00:18:35)
Factual error: When Starfire gets out of the totalled car, there is a road sign visible. It is a settlement exit sign, denoting the exit of Königsstetten and Zeiselmauer as next settlement ahead. Obviously the makers tried to be extra thorough nailing down their "outside Vienna" location. They started OK - both Königsstetten and Zeiselmauer actually ARE outside Vienna - but then they completely botched it: Not only did they use a German black-on-yellow "StVO Zeichen 331" instead of the Austrian "StVO Hinweiszeichen 17b" which has black writing on a white base with a blue rim and doesn't list the next settlement, but they also put 852km as the distance to the next settlement, which is more than the length of Austria west to east. The real distance is 3.8km.
Character mistake: After Jason falls from the skyscraper and is saved by Conner, he suffers some sort of PTSD. It is very hard to believe that a guy who fights crime all the time and fights people wanting to kill him so often (which includes Deathstroke beating and stabbing him like it was nothing) magically starts to suffer from PTSD just because he almost fell from a building. It doesn't make sense and feels forced.
Plot hole: Cadmus sets a brain-controlled Connor loose in public, without apparently remotely considering the possibility that anything might go wrong. When he regains control all they've got is a few goons onsite armed with regular ammunition - what did they think that would achieve? They know exactly what he's capable of. Either have kryptonite ammo or don't bother having anyone there at all.