Continuity mistake: When Tweety hits Sylvester on the paw with a mallet, the background behind Sylvester changes between shots.
Continuity mistake: When Tweety puts his blue hat on the tin of alum, Sylvester takes the tin and eats it including the hat. But when Sylvester is trying to capture Tweety by sucking him with a straw, Tweety is wearing his blue hat again.
Continuity mistake: When Sylvester (dressed as a cleaner) sneaks up on Tweety's cage to capture the bird, he puts his dustpan and brush next to the cage. But when we see Tweety standing next to the cage, the dustpan and brush have gone.
