Continuity mistake: Throughout the cartoon, the baggage car where Tweety and Sylvester are riding keeps changing position. In close-ups, it will either be 3rd, 4th or 5th behind the engine, but in long shots it's not there at all.
Continuity mistake: In the scene where Sylvester is sliding down into the bulldog who keeps punching him, there is a label on the side of the bulldog's cage saying "Warning Vicious Dog" that disappears when the bulldog slides down towards Sylvester.
Continuity mistake: Watch Sylvester's carrier in the scene when he is harassing the bulldog. You can see the carrier has a small front door and there are bars on the hole where Sylvester is looking out from, but when we see Sylvester later on, the door and bars have disappeared.
Continuity mistake: When the conductor swats Sylvester's paw after he attempted to grab Tweety, you can see the door to Sylvester's carrier is open, but when the conductor takes Tweety's cage, we cut to Sylvester and the door to his carrier is now closed.
Continuity mistake: In the scene where Sylvester tells the bulldog to be quiet when they are in their cages, you can see there is nothing behind them, but when we see the train about to go up a hill, we cut back to Sylvester and crates have suddenly appeared.
When we see the train during the opening credits, look at the carriages and you can see they have names of Warner Brothers staff on them: "Frizby" (Friz Freleng), "Hawley" (Hawley Pratt), "Ross" (Virgil Ross) and "Champin" (Ken Champin).