Revealing mistake: When Sylvester is on the run from Porky, he dives under the bed, but the cels of him diving were placed over the bed, so that he appears to dive on top of the bed and disappear.
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.