Other mistake: When Jack (Martin Short) is injected with the syringe, it's done in his butt cheek, through his pants, just below the waist of his coat. We then see later Margaret looking at photos taken during the scene, one of them shows the syringe injected through Jack's coat into his thigh/butt cheek area.
Other mistake: When Ricky confronts the mother superior in the dining room, Ricky raises the axe above his head and brings it straight down in one single motion. Both Ricky and the mother superior are facing each other. At that angle, it would have been impossible for Ricky to decapitate her. At best her head would have been cleaved in half, but not decapitated.