The hole in the curtain when Polonius is stabbed.
During Ophelia's lament, she reaches out with one arm to anyone who will help. In the next shot, the opposite arm is outstretched.
Polonius gets stabbed through the stomach/chest area, but when he is lying on the floor, he is bleeding from the back of the head and the wound where he is stabbed is non-existent.
During the "confrontation" between Hamlet and Ophelia, the shadow of a camera can be seen on the floor behind Ophelia as Hamlet begins to circle her..
Elsinore in Denmark is a very flat, not at like the hilly landscape portrayed in the film.