Continuity mistake: At the radio station, a saxophonist comes to Jack Bunny's desk to do his audition and you can see there are two pieces of paper on the desk. But when the accordion player does his audition, the pieces of paper have disappeared.
Continuity mistake: When Porky Pig falls asleep in his boat, he is wearing a red shirt. In his dream, a big fish cuts off the red shirt to cook a now naked Porky. When the dream ends and Porky wakes up back on his boat, his red shirt is still missing, and he's still naked. He should have still had his red shirt on.
Continuity mistake: When we see "Ben Birdie" on the stage at the start of the cartoon, he is surrounded by a piano, tuba and a drum, but when a mouse appears out of the tuba in the next shot, the drum has completely disappeared.