After being slapped around, when following Alek with her son, the blood on Margaret's lip disappears and reappears several times.
Alek is suspended upside down after the car crash, but the blood is shown to have run up his face, from forehead to chin.
In one scene Beau can be heard off camera practicing his trumpet then immediately ceases to appear on camera to speak with his mother. A bottle of valve oil is in his hand and one valve of his trumpet has been removed, presumably to oil. However, the time that elapses between his playing and appearance on camera is way too short for him to remove a valve from his trumpet.
Margaret's shoes disappear and reappear between shots when her son comes into the house and she is helping Jack.
When Alek is dying in the car, the amount of blood on his forehead changes dramatically, without any clear reason.