At the very beginning of the film when Ben strangles the
woman, she closes her eyes to show she has died, but then she opens them and then continues to blink until the scene fades.
When Ben has suffocated the little kid by putting a pillow on
his face, the body stops stumbling and is supposedly dead, but the chest still makes breathing movements.
When Ben plays the piano, his hands play out of sync with the
music.
At the beginning, Benoît says that four times a child's body
weight is needed to sink a dead child. However, at the bar where they drink Dead Baby Boys, Benoît asks René for the weight ratio needed to sink a child, to which René replies "Twice," and Benoit says, "Right!"
When Ben and Valerie play a duet together, he informs her to
play in 4/4 time when the song they play is actually in 3/4 time.
When Ben gets a holster for his birthday, he asks them to bring his revolver so he can try it out. But they don't bring a revolver, they bring him a semi-automatic pistol.