As the airship departs Frankfurt the Captain comments, "that poor band has been playing in the hot sun all day long" but the sky is cloudy, the pavement is wet from a recent rain and it's so cold you can see the breath of the actors.
When Merten crashes the glider into the lake, in the opening scene, Jennifer saves his life by performing mouth-to-mouth resuscitation. This technique was not invented until after world war 2.