When the cat opens up the "Be a Ventriloquist" box, the device inside is of an oval shape. In the next shot, when he takes the device out and puts it into his mouth, its shape has changed to rectangular.
When the dog lures the cat out using the cat costume and the cat knocks the dog down, the dog's fake tail disappears. It then reappears when he is confronted by the gang of dogs.
When the cat is writing "I hate dogs" on the fence, he draws an exclamation point and period. The period on the exclamation point is already drawn before he dots it.