Continuity mistake: When Jerry is dressing up Quacker in his schoolboy uniform, you can see the door to his house is only halfway open. But when he goes back into his house to fetch a mirror, the door is now wide open.
Continuity mistake: When the female duckling tells Quacker that he's cute, Quacker stands up from the rock he is sitting on and then runs under a branch. But when Quacker holds her arm, the rock has disappeared.
Continuity mistake: When Quacker shows Jerry the picture of the ugly duckling in his book, you can see the book is standing up, but when we see the picture in the next shot, the book is lying flat on the ground.