Continuity mistake: When the kitten says "cheese" when Sylvester is talking to the other cats after fleeing Porky's house, Sylvester slaps him and you can see there are footprints in the snow. But after a quick cut to Sylvester saying "listen!" we return to the cats and the footprints have disappeared.
Continuity mistake: When the cats are fleeing after hearing "Lassie", you can see that the front door has molding on it. But when the kitten lowers the doorknob to jump through the keyhole, the door is plain. In the next shot, when the drunk cat dives for where the doorknob was and hits the door, the molding is back, and when he falls to the floor in the next shot, the door is plain again.