When Agarn is adrift in the Balloon, a modern day paved road is clearly visible below him.
It has been established that FORT COURAGE is located on the frontier, in a desert area, yet when Agarn is adrift in the balloon, he passes over a heavily wooded forest area.
Col. Zeppel wears the Iron Cross on his tunic. The decoration had been instituted in 1814, but was allowed to lapse shortly after the Napoleonic Wars, only to be reinstated during the 1871 Franco-Prussian War (still a few years ahead of the 1866 time frame for the series).
Likewise, Agarn complains that Parmenter would have won the poker game between him and Zeppel except that "three Kaisers" beat Parmenter's hand. Kaiser, while an old title dating back to Charlemagne, had lapsed after the collapse of the Holy Roman Empire circa 1800 and would not be resumed until the accession of Wilhelm I of the German Empire in 1871.