Deliberate mistake: The Messerschmitts have painted yellow fronts, this wasn't done by the Germans until after Dunkirk. Christopher Nolan has admitted doing this deliberately so the audience could tell the difference between the Spitfire and the Messerschmitt during the combat scenes.
Continuity mistake: As the lone Spitfire trails and attacks the German bomber, he hits the right engine, which starts smoking but the trail quickly fades. He then hits the left, which smokes a lot. We then see the same attack from below, and from this angle the both engines are smoking equally badly.
Answer: When George got injured, before he died, Peter did not understand what the soldier had been through. In fact he was honest with him when he asked if George was OK the first time. After he got to Dunkirk he saw what happens in war. What people went through. He understood why someone would be shell-shocked and empathized with the soldier more. After that he wanted to spare the soldiers feelings and not make him feel worse.