Near the beginning of the film before James McAvoy heads up to the house for the evening meal, he is seen looking up through a skylight when a Lancaster bomber slowly flies overhead. At the time it was was supposed to be 1935. The Lancaster bomber did not fly until the beginning of 1941. And if it was supposed to be an Avro Manchester, then they only had two engines.
In the DVD commentary, Director Joe Wright reveals a lucky fluke that got caught on camera during the scene just before Robbie (James McAvoy) discovers the school girls massacre. At the point where he removes his helmet, the weather was cloudy. As he looks up the sky, the sun started shining, and then got cloudy again the moment he put his head down.