Continuity mistake: Before David gets stabbed in the pool, the hair of the guy with knife is messy, but during the stabbing it's brushed.
Continuity mistake: When the robot catcher nets in the stray robots including David they go in the cage of the moon balloon, where David is struggling to hold on to Teddy as the balloon rises into the air. Teddy says "l'll break David" and at that point Teddy slips through Davids fingers and falls to the forest floor. Teddy then immediately runs to the top of the hill where the balloon he just fell from has already sunk well behind that hill only just seconds after falling from it. The balloon has moved far too quickly.
Factual error: At the end of the movie when David jumps into to the water in downtown Manhattan, he lands in Coney Island. Coney Island is in Brooklyn, a long way from Manhattan.
Continuity mistake: When Monica first meets David, David walks out onto the carpeted floor and takes a step down to the wood tiled floor. He taps his feet on the wood tiled floor and then steps back up to the carpeted floor. He taps his feet again and turns around to face Monica. This is just before he tells her, "I like your floor." The mistake here is that Monica was pulled from the carpeted floor to the wood tiled floor before David walked in. So, David should not have had to turn around to face her because she would have been in front of him ... not behind him.
Plot hole: When Martin decides to antagonize David by eating spinach, he eats it from his plate. When they both eat the spinach frantically, they each eat out of a large bowl. Where did the second large bowl come from, and why would there even be a second one if they only need enough for three people?