During the film's opening titles, the camera zooms in on the werewolf's eyes which are clearly hazel. When Leon transforms in the jail cell, his eyes are initially Oliver Reed's natural color: pale blue. When he turns at stares at the old man in the cell with him while in mid-transformation, he is clearly wearing hazel-colored contact lenses like the ones shown over the opening titles. However, when he has fully transformed into the werewolf and moves towards the old man, his eyes are again clearly pale blue and remain that color for the rest of the film.
When Leon transforms in the jail cell, there is first a close-up shot of his hands growing fully covered in fur and twisting into paws. In the subsequent long shot, his hands look perfectly normal except for a few wisps of hair.
Pepe takes his wife's silver cross off the wall to make silver bullets to kill the werewolf. When she notices it missing, she refers to it as a crucifix. A crucifix specifically has an image of the crucified Christ hanging from a more realistic looking cross--this was just a plain, inornate cross. Indeed, in the primarily Catholic Spain, it would be more likely to see a traditional crucifix, rather than a plain cross, which is usually used by Protestants.
During the opening credits, which features a very tight close-up of the werewolf's eyes, the edges of the contact lenses can be clearly seen.
When the old beggar man is eating the scraps in the dungeon, you can clearly see his false beard coming off when he is chewing.
During the feast a band is playing which contains a modern, valved trumpet. Valves were not introduced on trumpets until the early/mid-19th century. Natural trumpets with no valves would have been historically correct.
When Leon is taken to the bawdy bar or nightclub, a modern (twentieth-century) roulette wheel is seen with gamblers surrounding it. Since the story takes place in the early to mid-eighteenth century, it would have been impossible for the roulette wheel to be there. Even earlier iterations of the roulette wheel only existed in Europe during the very late eighteenth century. Complicating issues further, roulette was outlawed in Spain throughout much of the twentieth century; and, roulette wheels were extraordinarily expensive. It would have been exceptional for a small village to have access to a roulette wheel even as late as the early-twentieth century. Suffice it to say that the roulette wheel and table's presence in the film is highly ahistorical.
The first 20 minutes of the movie are narrated by Don Alfredo. But since he did not witness the events depicted, there is no way he could have known what transpired.