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.
Deliberate mistake: Christopher Nolan admits he used a French destroyer instead of a British destroyer, for practical purposes (most people won't notice). The giveaway is that it has a closed bridge. British warships were built with open bridges till after the end of the Second World War, as can be seen by looking at HMS Cavalier (http://thedockyard.co.uk/explore/three-historic-warships/hms-cavalier/), which was built in 1944.
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.