Corrected entry: When in flight the jet-pack is always thrusting horizontally. There is no vertical component of thrust, and so there ought to be no lift supporting it & its wearer above the ground.
Corrected entry: With Neville Sinclair being so good at everything he does (acting, language, appearing to be who he is in general), why can't he dance? Just look at him with Jenny at the fancy restaurant: about as flexible as a bowling ball.
Correction: Oh, I know quite a few otherwise perfect men who can't dance. You just can't help being born with two left feet!
Corrected entry: When Jenny and her friend are talking on the set about playing scenery, Jenny's friend says "I love Hollywood". But she should have said Hollywoodland, since the place doesn't become Hollywood until Neville Sinclair crashes into the sign.
Correction: The name of the LA district of Hollywood has been known as Hollywood since the 1800's. The famous sign was originally an advertisement for a housing development within the district. It's the name of that development (Hollywoodland) not the name of the city district. So what Jenny's friend (and later Neville) says is correct. No mistake.
Corrected entry: Notice Neville Sinclair's English accent disappear aboard the zeppelin, switching into German? Indeed, "...it was acting."
Correction: Neville being an undercover Nazi posing as an English actor is one of the basic plot points to the entire movie. Naturally he would begin speaking with a German accent when he was around his own people. This is about as far from trivia as you can get.
Corrected entry: If the propaganda reel with the rocket-clad German troops is made in Nazi Germany then why does it show the German invasion of the United States as malicious and barbaric? It makes Germany look very sinister. Shouldn't it show America as a threat and Germany as a saint? Isn't that what propaganda is all about?
Correction: The propaganda reel was made to show German authorities how the rocket pack could be used to help Germany crush its enemies and take over the world. I'm afraid there is no way to sugar-coat that and make it look like Germany is the good guy. Either way, it's not a movie mistake.
Corrected entry: In real life, the flame discharged from the rocket would quickly set fire to the rocketeer's pants and boots.
Correction: The first time they set off the rocket engine Cliff touches the housing and comments that it is still cool. All metals are heat conductors - that must be a cool flame. Later, Cliff walks directly in the line of the rocket exhaust (when the statue crashes) without effect. Like I said, a cool flame which doesn't set fire to things. In the science fiction world, such things are possible.
Corrected entry: Neville Sinclair knocks out Jenny with a chloroform soaked rag - but this does not smudge her perfect crimson lipstick in the slightest.
Correction: Lots of lipstick is non-smear. It all depends on its' ingredients.
Corrected entry: There is one scene where a flying machine makes a crash landing, and as it skids along the ground on its landing "roll," you can see the cable on the ground which is attached to a vehicle (unseen) pulling the machine along.
Correction: I just watched this scene in slow motion. There are various markings on the runway that might be mistaken for towing cables, but that's all. No cable.
Corrected entry: When Sinclair summons his reinforcements, he yells: 'Sturmabteilung'. The term Sturmabteilung (roughly 'assault squad' in German) was reserved for the SA, which were - along with the Schutzstaffel (SS) - the two paramilitary organizations of the Nazi party. But in 1934, the SA was dissolved because of an alleged conspiracy. Commando troopers for this sort of mission would have been more likely picked from the Schutzstaffel, and certainly they wouldn't be called 'Sturmabteilung'.
Correction: First of all, the SA was not dissolved in 1934. Its leaders at the time were executed, and the organisation itself was shrunk, but it was not disbanded until the end of the war in 1945. Secondly, the SA also had several military units, including their own elite troops, known as SA-Standarte Feldherrnhalle. It is therefore not impossible that SA soldiers were the ones assigned to Sinclair's command.
Correction: Passenger planes don't have a vertical component of thrust, either. The lift is caused by a difference in air pressure on one side of the wings compared to the other. Who's to say the design of the rocket didn't use itself and/or its human component to achieve lift due to differences in air pressure? As we don't have a working version, it could be argued that "it just works," just as well as you could argue that there's no "vertical lift."