Continuity mistake: As the priest descends down the rope ladder into the church for the first time he is handed a lantern by his friend. In the next shot the camera is looking down and you can see that the friend is holding the lantern. Then the next person goes down the same rope ladder and the friend is not holding the lantern, where did the lantern go?
Continuity mistake: In the original film, several priests discuss who should do the exorcism. Merrin was chosen because he had experience (supposedly this film), and that the exorcism lasted a month and it nearly killed him. In this film, the exorcism appeared to have lasted only a few hours and, although I'm sure he was in very real danger, he didn't seem the worse for wear after it was over.
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.
Other mistake: Both times that Father Merrin ventures into the sacrificial chamber beneath the church, as he is walking down the flight of stairs, the rock wall behind him is well lit as if there were another lamp sitting right in front of it. In the shot immediately after it, the rocks are in total darkness, even though he has not moved much. He is already far from them in the first place, so the light could not have reached the rock wall and illuminated it so brightly.
Revealing mistake: After the power goes out at Sarah's hospital, she lights a candle. While she's walking along with it, it's obvious it's not made of wax. It looks like it's made out of plaster.
Continuity mistake: As Merrin and Sarah are sitting on the bed about to kiss, the opening of the mosquito net around Joseph's bed changes several times.
Continuity mistake: During Merrin's first conversation with Jeffries, in the Kenyan bar, the liquor bottle on the table rotates several times.
Revealing mistake: When entering the tomb there are quite a lot of shadows on the walls and statues. We are supposed to be in a sealed tomb. Where is all the light coming from? You can tell by the shadows that light is coming from multiple locations, not just the priest's lamp.
Continuity mistake: In the opening scene, when the priest first sees the demon idol laying on the ground it is a few inches below the soldier's hand. When he reaches down to take it, the idol is grasped in the hand.
Revealing mistake: The two workers to the right of the entrance are fake digging. Their picks never touch the ground. (00:27:20)
Answer: Weapons on statues in conventional churches point upwards in veneration. The weapons here are pointing downwards in an offensive gesture.