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.
When dropping Jennifer off at the airport, they're in a rusty red Toyota Tercel, but while driving home with the dog, John is driving a Honda Accord.
In the scene where the kids get off the school bus, at first you see two cars in the driveway, but in the next shot looking at the house there is only one car.
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.
When John gets a cup from the cabinet, the cabinet is open but in the next shot it's closed.
Marley's features (ear shape, head shape, color) keep changing during the duration of this movie revealing that more than one dog was used.
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.
When Debbie the dogsitter is startled by Marley in the shower, the hem of her body suit is visible pressed against the shower curtain.
Relatively early in the film, in the early 1990s, John Grogan (Owen Wilson) mentions taking a trip to Disney World and a 2000s Walt Disney World sign is shown.
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.
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.
Set in the early 90s in the beginning, the scene where John drops Jennifer off at the airport, a sign is seen for Airtran, which was called Valujet until 1998.
Early in the film, while still set in 90's, when Jen is leaving and they are at the airport, a Nissan Murano is clearly visible in the background. The Murano was not produced until 2004.
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.
During the snow day scene, there was a 2001 Honda Odyssey in the driveway when the kids look at window, while playing outside the 1998 Model remains present.
In the first scene in the their house, Jennifer is reading John's column, an ad on the back page has discount misspelled "diccount".
After John picks up Jen from the airport and Marley destroys the garage, Jen is petting Marley and calls him Miley not Marley.
At about 1 hour and 28 minutes into the film, John picks up a newspaper, the New York Times, and opens it to find an article written by his friend and coworker, Sabastian. The article is entitled "With U.S. Training, Colombia Melds War on Rebels and Drugs." In the first sentence of the piece, "guerrillas" is misspelled "querrillas."