Adults, especially parents, are terrified of a murderer who targets children--yet children are shown playing outside unsupervised and talking to strangers without reservation.
In one of the police crack-down scenes, the German bar hostess keeps referring to the uniformed police officer in charge as "Herr Hauptmann." The English caption translates this as "sergeant." But, in actuality, "hauptmann" is equivalent to "captain".
When the gangsters find M in the attic and turn on the light, the clock behind M moves back 5 minutes from 11:55 to 11:50.