At the beginning of the movie, when Pearl dances for the farm animals, climbs on top of the bales and raises the pitchfork above her head, the tines change sides - from Pearl's left side, to her right side in the following scene.
During Pearl's first meeting with the projectionist, the two share a toast with their wine glasses; Pearl is initially seen toasting the glass with her right hand, but in the subsequent shot, the glass is in her left hand.
Although set in 1918, the Ford Model T used in the film wasn't produced until 1926-27.
The projection booth only has 1 machine. It would be necessary to have two projectors in the room at that point in time to screen features.
The movie "Palace Follies" that Pearl goes to see at the theater has accompanying sound and music. Given that "Pearl" is set in 1918, this is about eight years too early. Although experimental short films sound had been shown as early as 1894, there were no feature-length movies with synchronized sound before "Don Juan" was released in 1926. Of course, given Pearl's troubled mental state, she may have simply imagined the music.
The term "x-factor" said during the audition scene was not common slang in 1918. Merriam-Webster states it's first use was in 1930.
Mitsy tells Pearl to "break a leg" before her audition. This was 1918, several years before the phrase started being part of the theater lexicon, and even longer before it would have reached the boondocks where they lived.
Throughout the scene in which Pearl kills her father, the sleeve on her left arm slowly falls down from her elbow to her wrist.