The characters constantly refer to the province Jerusalem is located in as "Palestine". At the time the film is set (AD 30's), Jerusalem was located in the province of "Judea". Judea would not be called Palestine until Emperor Hadrian renamed it ("Syria Palaestina") in 135 AD at the end of the Jewish Revolt.
The Emperor Tiberius' wife, Julia, puts in an appearance complaining about Diana being considered "too good for Caligula" and Tiberius mentions his "30 years with Julia". Actually, his wife, Julia, the daughter of his predecessor, the Emperor Augustus, had been permanently exiled by her father for lewd behavior long before Tiberius even became Emperor. By the time "The Robe" opens, in the last years of Tiberius' reign, Julia had been dead and forgotten for decades.
While Miriam and Marcelus are chatting under the tree and Miriam is weaving a basket she runs out of basket material but the next long shot she has a long section of it, not once but twice, without picking up another piece of weaving material.
Caligula is depicted in this movie and its sequel "Demetrius and the Gladiators" as persecuting Christians. However, he reigned from 37 to 41, while 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).
Justus tells Marcellus that his grandson, who was born crippled and whom Jesus healed, was named for "Jonathan of old", who (Justus claims) was also born crippled. In the Jewish community, "Jonathan of old" would be the son of Saul, first king of Israel, and the best friend of David, Saul's successor. Jonathan was not born crippled; he had a son, Mephibosheth, who "was lame in both feet", but that was the result of an accident when he was 5 years old (2 Samuel 4:4).
In the sword fight between Marcellus Gallio and the Roman commander, there is a shot of Gallio holding his sword to the commander's throat, while the commander is on the ground. The sword is an obvious prop that is rounded on the end like a spatula.