After Alex fires Mrs. Connelly's harpoon the rope is stretched out. In the next shot the rope is wrapped up on the harpoon.
In hospital after the shooting Nancy's hand on Alex shoulder moves about between shots.
At the housewarming party, there are holiday wreaths on the front door of the house at the start of the party, but they are gone later in the evening.
When Mrs. Connelly feeds Alex a Bugle chip, she dips it in onion dip, but in the shot of her hand putting it into his mouth (and in the close-up shots of Alex) it is no longer covered in dip.
Mrs Connelly returns the wine to Alex. After the door is closed, he has the bottle in his left hand. On opening the door the bottle is in his right hand.
An emergency-room doctor examines a gunshot-wound patient without wearing gloves. No trained doctor would make such a mistake.
The parrot Little Dickie is able to fly around the Duplex, but in close-up shots it is apparent that the bird's wings are clipped.
The Apple logo on Ben Stiller's characters computer is upside down. All Apple Laptops are built in such a way that when the screen is open. The illuminated Apple logo on the lid of the laptop is stem end up.
When the woman at the housewarming party says "I got an award for this one" she clearly doesn't say "one".
Mrs. Connelly's accent fluctuates between Irish and Scottish.