Set in the early 90s in the beginning, the scene before John drops Jenny off at the airport, you can see an American Airlines Boeing 737-800 flying over the airport. The Boeing 737-800 was not operated by AA until 1999.
At about an hour and a minute into the movie when John gets the newspaper from the drive and reads Sebastian's article on the drug war in Columbia; if you hit pause on the newspaper, you find the date to be April 5, 2001. However, the article is about Columbia requesting more funding for the war on drugs and on the second column there is a quote from "President Clinton." Bill Clinton had already been out of office for almost three months on April 5, 2001 and George W. Bush was president.
In one scene they have two cars a Honda and another car. But in one shot, instead of the Honda in the driveway there is a Volvo. Then in the next shot it is back with the Honda and the Volvo doesn't appear till a couple of scenes later.
In the scene where John and Jen are choosing a puppy, you can see that Jen is holding "clearance puppy". The next time you see her she is not holding a puppy, then she is holding the puppy again, and then she isn't.
This goof is also in the trailer. When Marley is being taken to the vet, he is trying to get out of the car. He starts walking and his hind legs are in the car but in the next scene they are outside the car. They are then in the car and back out again.
The first outside shot of Debbie the dog sitters car shows a burgundy older model Volkswagen Cabriolet. When the car is shown through the window in the next shot it is now a newer model Volkswagen Cabriolet and a brighter color red.
When the 'clearance puppy' is in the car on his way to home, he doesn't have a collar. However, when they reach home, he does. Again, at night, when Marley cries and howls till John takes him to his room, he (Marley) isn't wearing a collar.
When John and Sebastian are taking about Sebastian going to Columbia, John has his bag over his shoulder and removes it. The editor comes out and tells John he has to cover a methane leak, and he has the bag is on his shoulder and removes it again.
(at around 1h 35 mins) The position and direction of the dog collar changes from one side to another. At first the direction of the buckle is clockwise and after a while the direction of buckle is anti-clockwise.
When the main characters are walking down the beach and are shown from in front, you can see the stirred up sand from dragging the camera along. When the shot changes to behind, this is not visible, nor when they are shown from the side. These three viewpoints keep rotating, making the trail appear and disappear repeatedly.
About two-thirds of the way through the picture, West Palm Beach Police show up at the beach, responding to the dog pooping in the water. This beach is in the jurisdiction of the Palm Beach Police, not the West PB Police.
When Jenny returns from Gainesville, John picks her up from the airport in what it seems to be a rainy day. Even though the water is pouring on their car, the men walking in the distance are obviously not rained upon and the asphalt is dry.
When John and Jenny first go to look at puppies, the breeder tells them that they'll have to wait three weeks until the puppies are weaned. This would put the pups at about four or five weeks old, when they should weigh about 4 pounds. However, the size of the puppies used in the scene puts them closer to 10 or 12 weeks of age.