Factual error: In the very beginning of the film, the priest is walking through a battlefield that contains the bodies of Roman soldiers (they are wearing distinctive Roman uniforms and armor), which dates the battle no later than 6th century AD. He is holding a rosary in his hand, but the rosary wasn't introduced into the Catholic Church until the 13th century.
Factual error: Father Gionetti gives Father Merrin a copy of the Rituale Romanum (Roman Ritual). (The administration of exorcism is in Title X of the Rituale Romanum.) The title of the book embossed on its leather cover is "Roman Ritual", in English. Later in the film, Father Merrin asks Joseph to help him by reading passages of the book, and we are shown a couple of printed pages, the text of which is also in English. The Rituale Romanum would have been printed in Latin in 1949, when the film is set, because non-Latin translations of liturgical texts were not authorized until the promulgation of the Sacrosanctum Concilium (Constitution on the Sacred Liturgy) by Pope Paul VI on December 4, 1963, as part of the Second Vatican Council.