The Count's hat on the hat rack changes positions between shots, despite not being touched throughout the scene where he talks about his plan to con Kouzuki and Hideko.
Sook-Hee's skin tone changes from between tan, to pale, and back again throughout the movie.
When Sook-hee goes to Fujiwara's room after he arrives at the mansion, he is shown in the background of shot tying his bow tie. Whilst Sook-hee begins to slide the door shut, Fujiwara lowers his right arm. When the scene cuts to Fujiwara inside the room after the door closes, both of his hands on the bow tie.
During the dress-up scene, Hideko's left hand is positioned outward when she compliments Sookhee. When the scene repeats later in the movie, Hideko's hand is positioned inward.
In the opening shot, some soldiers scare off a group of kids and they begin running away in the opposite direction. There are eight kids shown initially, but when they run past Sook-hee in the next shot, only four of them are shown.
Mercury is a slow (typically chronic not acute) poison that will kill victims over a period of days not within minutes. Metallic mercury in the tiny amounts presented would not be lethal at all in the circumstances shown.
The Count uses a propane gas lighter. That was impossible in 1930.
In the basement torture room, the penises are circumcised. Circumcision did not become a standard practice in Korea until later in the century, and essentially no Japanese or Korean men would have been circumcised during the time of the film's setting.
Main character looks at her watch after giving opium to "count". It's a seiko solar watch! Says so on the face.
Because the actors have dark brown eyes, several times throughout the movie you can see filming equipment in their eyes.
There's a boom mic visible above Count Fujiwara in the second occurrence of the scene where the Count jumps from his chair and remarks how beautiful the Lady is.