Continuity mistake: When the orange cat attempts to escape with Tweety, he is hit in the face by an iron and slides back down the stairs. At this point, you can see the door to the exit is open, but when Tweety escapes a little later on, the door has gone.
Continuity mistake: When Sylvester and the orange cat take Tweety Pie's nest, you can see the outside of the nest is covered in snow. But when the two cats reach the bottom of the pole, the snow has gone.
When Sylvester hits the orange cat on the head with a mallet, look in the background and you can see a box with text printed on it advertising "Friz - America's Favorite Gelatin Dessert" - a reference to the director Friz Freleng.