While Robert Taylor is making his speech in the arena, the dead bull is now missing.
(at around 2h 40 mins) When Nero runs from the arena to his apartments inside the palace, it's sunny and bright, but moments later when he goes onto the palace balcony to observe the furious crowds of Romans, it's night time.
(around 2 hours, 8 mins) When Petronius and Eunice are reclining at the dinner table, there is a decoration of flowers behind them. However, in the next shot when Petronius starts talking to his guests, the decoration has changed and now there is also a pedestal with a cooked pig on it. On the next shot after that, the pedestal with the cooked pig is no longer there and only the flowers remain.
In a conversation between Marcus and Petronius, side shots show Petronius' arms in his lap, whereas front shots show him holding a goblet at chest level.
As Petronius is committing suicide, one of his friends lays his head down on the table in grief. From one angle, it's a friend on the right side of the table. From a similar angle, it's a friend on the left side of the table, and it does not appear to be the same actor in the scene.
Whilst the arena where the Christian executions take place has an elaborate ornament at its center, no such center would ever actually have such an ornament, as it would block the view for patrons on the other side.
Marcus Vinicius is angry because the Emperor will not allow him to bring his legion into the city of Rome. Since the early days of the Republic a military commander was forbidden to bring his troops armed into the city of Rome.
At the beginning of the film at the head of the marching army are several standard bearers carrying eagles. There was only one eagle per legion.
Vinicius is described as both a victorious Military Commander and a Military Tribune. In fact, a Military Tribune was a junior staff officer in his mid 20's. There were typically 10 in every Legion of 6,000 men. They were basically aids to the Legion's C.O., the Legate.
Nero, in fact, died four years later in history than shown in the film. However, the events surrounding his death (citizens revolting, committing suicide with some outside help) are true to fact.
In the opening shots of the film, as the camera rises above the march towards Rome, we briefly see that the wheel tracks behind Robert Taylor's chariot are just a few feet long, indicating that the whole parade has not reached this point on the Appian Way after miles and miles of marching, but rather just a few seconds after the director called "Action".
Obvious "blue spill" on characters in several scenes, revealing the use of the blue screen process to composite actors in previously shot footage. Examples of this include the close-ups in the chariot scene and the scenes of characters overlooking the burning of Rome.
The lions are attacking obvious dummies in several frames.
When Nero plays his lyre as Rome burns, the seams and stitching of the night "sky" behind him are clearly visible.
When the lions are first seen emerging from the cage onto the field, it's possible to see through them.
Both the main city backdrops of Rome AND the model of Nero's planned New Rome (or "Neropolis") show Rome at its architectural height in the 4th Century AD. In fact Nero's plan for his New Rome - which he actually built - was to replace the entire center of the city with a sprawling private park and gardens with his huge "Golden House" in the center.
Lygia notices a big sword after Ursus' big fight with the two attackers. Romans used short swords and javelins since Vandals had yet to invent the stirrup several centuries later, which enabled heavy cavalry, not just scouts and archers.
Though it is much debated, the cross may not have been used as a Christian symbol until the second century AD, some decades after the setting of Quo Vadis. In any case, it makes no sense that the Christian characters are secretive about the fish symbol, yet they hang crosses in their homes where anyone can see them. (And if the fish symbol was a secret, how did the pagan fortuneteller and the non-Christian chief concubine in Nero's harem know about it?)
Chess was unknown in Europe until at least the seventh century, yet we see soldiers playing.
Whilst it is true that variants of chess have existed for over 2,000 years, the chess set being used in the film is far too modern. The set shown uses what are apparently modern pieces, which didn't exist until the few centuries before the film's release.
When Lygia calls out to Marcus during the burning of Rome, her lips do not move.
Several lyres and classical harps are seen, but the dubbed soundtrack uses the contemporary pedal harp, which has a proper sound box, so sounds much richer than a Roman instrument. Modern harps also have many more strings than the instruments seen, which in addition would have been tuned in a different mode (scale), so would sound strange and out of tune to today's ears.
The scene where Nero is on the balcony and soldiers are trumpeting, a modern water tower is shown the the background. Rome had the aqueduct not a water tower.
When Nero shows off his model of the new Rome, Petronius asks "But what of the Rome that has stood for 1000 years?" Rome was dated "Ab Urbe Condita" from the city's founding, reputedly in 753 BCE. Petronius would have been careful not to insult Nero by exaggerating, so he would have said 800 years.