Revealing mistake: When Igby and his brother are removing the plastic bag from Mimi's head, her eyes blink.
Continuity mistake: When Igby's dad breaks the shower door, he breaks it with his right arm. The next shot has him cradling a badly cut and lacerated left arm.
Continuity mistake: When Igby is at his Godfather's party, and is drinking from the wine glass with cigarette ash in it, the glass changes hands. When the ash is put into the glass, he is holding it in his right hand, then the camera angle changes and it is now in his left hand, where it remains for the rest of the scene.
Continuity mistake: When Igby and Sooki are eating ice cream near the end of the film, the amount of ice cream on Sooki's spoon changes throughout.
Continuity mistake: In the death scene at the beginning, a spoon drops to the floor when the mother dies. At the end of the movie, in the return to the death scene, the older son is feeding the mother the poison and then sets the bowl and spoon aside. The mother never has the spoon in her own hand.