Continuity mistake: When Elmer slams the front door after being tricked by one of the cats, you can see that the door has a handle and lock on it. But when the cat dressed as a salesman leaves Elmer's house later on, look at the door and you can see that it has now changed to a door knob and keyhole.
Continuity mistake: When Elmer is trying to explain the money situation with Sylvester, you see that Elmer is sitting on his chair. After a quick cut to Sylvester nodding, we cut back to Elmer and he is next to Sylvester, but his chair is nowhere in sight. In the next shot, he is back on his chair again.