In one scene Mark hides behind the front entrance doors of Apollo House, watching the kids through the letterbox, then Toni opens the door, accidentally knocking him over. In this scene the doors are covered with gray panels, however at the beginning of the episode when Mark first encounters the kids, we see that the doors are clear glass, with no letterbox.
When Jeremy dances to his track in his room, his POV shows the reflection in the mirror; the background behind him changes position as it cuts to a different point in his dancing. This happens twice.