When Otis Driftwood is sitting on top of the steamer trunk being pushed by the porter and has the porter stop in front of Rosa's cabin so he can go inside, he's wearing a hat. In the next shot, from inside Rosa's room as he enters it, he's not wearing a hat. When he leaves Rosa's room and goes to Mrs. Claypool's room he's still not wearing a hat, but after he leaves Mrs. Claypool's room and we cut to the next scene which is Driftwood entering his stateroom, he's wearing a hat again.
At the beginning of the opera, Gottlieb (having just called Groucho Marx a "Schweinehund") is in the balcony with Mrs. Claypool. The next shot shows the conductor calling the orchestra to attention, and in the background Mrs. Claypool is now alone in the balcony.
Chico Marx plays a turn at the piano surrounded by boys and girls. When he gets up, there is a boy there dressed in a light shirt with nothing around his neck. Harpo Marx comes to sit down, and the same boy now has a darker shirt with a neckerchief.
When Otis Driftwood arrives at his "suite" on the boat, the first time he enters, the room is small. And in the next shots of the room, we see that the room is a little bigger. The pipe that's in the room is closer to the back wall in the first version of the room, and more in the middle in the second version. There's also a chandelier in the big room that doesn't appear in the small one.
The elevator operator uses the old movie trick of rising from a squatting position to make it appear that the car is rising from the ground floor. Unfortunately, the white elevator certificate over his left shoulder remains motionless the entire time.
During the overture to "Il Trovatore" when the orchestra members turn their pages and immediately launch into "Take Me Out to the Ball Game" you can see that their score consists of two pages of sheet music side by side. If they were really following it they should have continued playing the correct music from the opera on the left-hand page before they came upon the baseball anthem, which sits atop the right-hand page where Harpo Marx placed it earlier.