Continuity mistake: When the cat's tail is in the sink's plug hole, Ralph is standing by a switch and you can see there is nothing on it, but in the next shot the word "Disposal" just appears from nowhere, and when the cat is picking up his fur from the plug hole, the wording has gone.
Continuity mistake: When Ned is pretending to be invisible, he pulls out one of the cat's whiskers and there are now five whiskers on the cat's face. But when Ralph is on the champagne cork and is swallowed by the cat, the cat now has six whiskers again.
Continuity mistake: When Ralph is walking up the steps to his apartment, he has a present in his right hand. The present disappears when he goes through the doors, but re-appears when he enters the kitchen.