Continuity: When Tiberius and Caligula are in the three-story sex building (noted by the man with stilts), first Caligula goes down, following Tiberius on the elevator. Later, they are talking at the bottom, then back on top of the building, then back at the bottom.
Continuity: Caligula is stabbed many times by the guards, yet his robe shows no signs of damage.
Anachronisms: There is a huge map of Europe and the Mediterranean on the wall. It is a modern, extremely precise map, much more precise than available at the time.
Anachronisms: Most of the penises seen in the movie are obviously circumcised. However, circumcision was not practiced in Rome.
Factual errors: Nerva is shown killing himself in the year 37 AD. He actually died around 33 AD.
Continuity: In the prologue, Caligula is clean-shaven. However, in the very next scene, after the credits, he suddenly has a beard. The two scenes supposedly take place within the same day.
Factual errors: Charea proclaims Claudius emperor after killing Caligula and his family. In reality, it was another Praetorian who proclaimed Claudius as emperor; Charea had been planning to kill Claudius as well to eliminate the entire imperial family.