Continuity mistake: When Ralph is ready to surprise Alice, there is a switch by the door. But when Ralph tells Ned to bring in the cake, the switch has gone.
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 the cat jumps up and stops next to Ned, you can Ned is well away from the cat. In the next shot, Ned is much closer to the cat than he was previously.
Continuity mistake: When Ned and Ralph are at the clockwork toy, the toy has a lever on its side. But when the toy comes to the cat, the lever is missing.
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.