Revealing mistake: After the alley cat has put Fernando in his mouth, Speedy runs back and forth before hitting the cat on his foot. If you watch carefully, you can see nothing hits the cat's foot at all, but he reacts as if in pain.
Continuity mistake: During the slow-motion sequence, Speedy slams the mallet on the cat's paw and leaves the mallet on the ground when he rescues the mouse. When he puts a stick of dynamite on the cat's tongue, the mallet has gone.