Factual error: At the end of the classroom scene, Indy gives Marcus the Cross of Coronado. There is a pile of books on Indy's desk that he laters picks up and takes with him. One of the books (with the whitish spine, under the red book) is "Living Egypt", written by Paul Strand and James Aldridge. This book wasn't published until 1969, over 30 years after this movie takes place (1938). (00:15:20)
Factual error: Jehovah is spelled Iehovah in Latin. However medieval languages (Latin or vernacular) had neither official rules nor the letter 'J'. It was not until the sixteenth century that the French humanist Pierre de la Ramée proposed to use the 'J' as a separate letter distinct from the 'I'. The 'J' could therefore not have been a trap at the time the test was constructed. (01:46:20)
Factual error: German Fighters shown in the film are in fact Pilatus P-2 (Swiss-made aircraft built after World War II). The appropriate fighter of the period would be the Messerschmitt 109. (01:17:00)
Factual error: During the motor boat chase through the canals of Venice, Dr. Schneider pilots the vessel between two ships, one identified as the John W. Mackay of London. Although the Mackay was commissioned in 1922, it was stationed at Halifax, Nova Scotia for service in the North Atlantic. It was deployed for the first time to the Mediterranean in 1942 - four years after the purported time of the movie.
Factual error: All through the film, SS-Standartenführer Vogel (Michael Byrne) is seen wearing the Iron Cross in second place beside his SA Military Sports Badge. This is totally wrong. As the Iron Cross is a Military Award, it would be worn over or to the right of any other badge on the pocket. Also presuming this is a 1914-18 Iron Cross, he should then be wearing a ribbon bar over his left pocket displaying his WW1 Honor Cross Ribbon along with long service ribbons, etc. American Law specifically allows the accurate wear of uniforms in movies (see http://uscode.house.gov/uscode-cgi/fastweb.exe?getdoc+uscview+t09t12+443+8++%28%29%20%20AN (subsection f)) and there is no reason to believe (without evidence) that German Law forbids the accurate portrayal of uniforms. (00:53:20)
Factual error: The evil SS officer is addressed as "Herr Oberst" (Colonel) throughout the movie. In the SS, however, this rank was called "Standartenfuehrer". (00:56:40)
Factual error: When Indiana's father uses the plane's gun, he shoots out the tail. This was not actually possible, as the planes had a safety mechanism to stop people doing this. In fact we can see that the Lewis gun he is using is mounted on a Scarff ring, which prevented the trigger of the gun being activated when the barrel was pointed at the aircraft's tail surfaces. (01:17:40)
Factual error: Indiana Jones checks his dad's grail diary to find the picture of the stained glass window in the church in Venice. With the book open to that page, the picture is on the left-side page, and under Indy's thumb on the right-side page is a snippet of an article about marriage statistics. The article mentions statistics up to and including the year 1943. Problem is, the movie takes place in 1938. (00:27:00)
Factual error: When Hitler signs the diary, he writes in a modern way of writing. But in the late 30s there was a different style of writing called "Suetterlin". The letters looked quite different from nowadays. (01:10:25)
Factual error: When Hitler signs the Grail Diary, in the screen and VHS release he signs Adolph, not Adolf. This mistake was corrected in the DVD release. (01:07:35)
Factual error: There are no catacombs in Venice! There are also no natural petroleum sources - but that seems secondary to the fact that the entire city is just below sea level - there are no sewers, no subways - nothing underground.
Factual error: When Indy and Marcus first arrive in Venice on the boat the dock they alight on to has tactile pavers (for the visually impaired) installed on the platform. These weren't invented until 1965, some 25+ years after the scene takes place.
Factual error: When Indy and Marcus fly to Venice, the route map shows them going from New York to Botwood, Newfoundland, then from Botwood to the Azores to the continent. There were two transcontinental air routes at the time across the North Atlantic: one from New Work to Botwood to London, and one from New York to the Azores to the continent, but none from Botwood to the Azores. (http://people.hofstra.edu/geotrans/eng/ch3en/conc3en/earlyairnetworks.html).