Soldiers shoot rifles (obviously loaded with blank ammunition) inside the villa at almost point-blank range at each other, yet tend to somehow aim and fire slightly wide of the nearby target each time (but still score hits). Presumably firing wide like that to lessen the danger from the far ejected visible hot gunpowder from the rifle barrels due to the very close range.
When Nero's ghost levitates the sword in the teaser, wires are clearly visible holding it.
Tony and Doug light a candle to provide light in the catacomb. A few moments later, the two pass by the table and the candle is out, yet the room remains illuminated.
The General wants to try to get Tony into the Time Tunnel complex by initiating a transfer after an electric shock they have transferred to him drives the ghost of Nero from him. The General seems confident that they can do it, but throughout the series they are constantly stressing that they have no control over where Tony or Doug end up & have to go looking for them in history, based on their radiation readings.
This is set in 1915, but the German soldiers are wearing "coal scuttle" helmets, which were not widely available until 1916.
The German steel helmets are the M35 pattern worn by Germans in WWII.
In 1915 the Italians were fighting the Austro-Hungarian army in the north of Italy, the Germans did not send troops to the Italian front until 1917. The late 1917 offensive by Austro-Hungarian and German forces was a major defeat for Italy. Future important people participated in that battle, such as the future Pope John XXIII, Benito Mussolini, and Erwin Rommel.
When the books are flying off the bookcase when Nero's ghost is on a rampage in the study, you can visibly see a stick pushing a row of books off the shelf from behind the bookcase.
During the fight with the German, about halfway through. The Count, Tony and the German soldier has light hair, his stunt "double" has dark and is way taller than the actor.