Near the end of the movie when Peter goes to the house of Messalina to see Demetrius, she throws wine on Peter. The wine stains Peter's undergarment on his shoulder and drips on his draped outer tunic's chest. When the camera cuts to another angle, Peter now has a smaller wine spot only on his shoulder with no dripping stains on the chest of the outer tunic. When the camera switches back, Peter again has both stains (shoulder and chest) prominently along the front of his outfit.
Caligula was murdered by several conspirators in an underground passageway, not in an arena by one man. Claudius was found hiding behind a curtain, certain he would be murdered as well since he was Caligula's uncle, before being proclaimed the new emperor. Finally, there is no mention of Caligula's wife and daughter, who were also assassinated.
Caligula was emperor of Rome from 37 to 41 CE. The apostle Peter is known to have been incarcerated by Herod Agrippa, who reigned from 40 to 44 CE. When he came to Rome is unknown, but this must have been later, only after there were followers of the new religion there (after 50 CE). So, Caligula and Peter were never in Rome at the same time.
Caligula is depicted in this movie and its prequel "The Robe" as persecuting Christians. However, he reigned from 37 to 41 (he's killed near the end of this movie), whilst Christianity was still a nascent religion with most of its followers in the eastern Mediterranean. The first mention of Christians from the perspective of the Roman government, according to the Roman historian Suetonius, wasn't until the reign of his successor Claudius (reigned 41-54). The first major incidents of persecution of Christians did not occur until the reign of Nero (reigned 54-68).
Claudius (Barry Jones) doesn't stammer once. As is well known, Claudius had a severe stammer and was widely (and wrongly) despised as an idiot.
When Claudius and Messalina were interviewing Demetrius on his Christian beliefs, the parrot on the perch next to Messalina's couch was a sulfur crested cockatoo, indigenous to Australia. Australia was unknown to the western world in the first century AD. Therefore, such birds would never have left their native habitat, much less be captive pets in Ancient Rome.
A common misconception was that half the gladiators in the arena would be killed. In actuality, there were many times that the losing gladiator's life was spared. This was due, in part, to him, or her, displaying great skill and courage, as Glycon tells Demetrius in the first half of the film. Gladiators were also very valuable to their owners. Emperor Claudius, who followed Caligula, was known for showing no mercy, even when a losing gladiator put up a good fight. In this film, Caligula could have granted clemency, but decided not to. It can be seen as a plot device to show him to be a ruthless leader.
When Caligula gets the robe from Demetrius, he takes it to a man chained to two pillars. To test its "life" powers, he has a Praetorian Guard stab the chained man. When the Praetorian pulls the sword out of the man, it is clean and has no blood on it.
When Demetrius stabs a tiger several times on its light-colored underside, there is no blood either on the tiger or on the dagger. There is also no blood on the dagger when Demetrius later stabs one of the other gladiators and pulls out the dagger. No blood appears on the gladiator, either. It is obvious that the blades on these prop daggers retracted into the handles upon stabbing.
In the beginning of the movie, Claudius presents Caligula a report from Pontius Pilate. When Caligula became emperor (37 AD), Pilate was already dismissed of his charge as Prefect of Judea (26-36 AD).
The film is set in AD 53. One of the statues surrounding the Emperor's throne is Michelangelo's David which wasn't finished until 1503.
In the beginning of the arena scene as the camera pans left of the royal box, you can see an electric pole and wires in the background with the trees.
Whilst Caligula (Jay Robinson) is talking to Claudius and Messalina about the death of Marcellus and Diana, he draws a dagger and plays with Claudius by threatening to stab him with it. When he puts it back in its sheath, sheath and dagger inadvertently turn upside down sticking out at an odd angle and irritating Jay Robinson, who tries twice and finally rights the sheath at his waist.