During the opening montage of the various pets acting up while their owners are out, Mel is shown barking at a squirrel that is in a tree outside his window. At the end of the movie as the camera zooms away from the building, Mel is shown jumping around one floor below Max's apartment, which is 7-8 stories up. There is no tree outside of Mel's window (or any tree at all).
When Mel's owner is leaving, the front door is white and there is wall-to-wall blue carpeting on the floor. In the next scene, the wall-to-wall carpeting has gone and the front door is now blue.
When Max is waiting for Katie to return and sitting in front of the door near the beginning of the movie, a coat rack can be seen on the wall to the left of the door with a blank wall above it. Later, the guinea pig Norman comes through an air vent that has miraculously appeared above the coat rack.
Katie's coffee table disappears and reappears.
When the dogs are in the park, Max's leash keeps changing position on his collar, from one side of his "Max" name tag to the other.
When Duke drags Max from the park by his leash, Max is seen being pulled through tight railings in an apartment block staircase. For Max and his leash to be pulled between the railings, Duke must have passed through first. This is impossible since Duke is several times bigger than Max.
Sweet Pea switches on a fan to fly at speed indoors, but upon moving closer to the TV she leaves the air-stream so the bird should have crashed into the screen.
Shortly past the halfway mark, when Max and Duke are on the river and Max is endangered by his insufficient swimming skills, Duke throws him a life preserver and uses his mouth to reel Max in. But since Duke is essentially using his mouth as a hand, every time he releases the rope to change grip, Max should be pulled back, away from the boat, and should even be pulled too far back for Max to bring him in. This never happens; Max stays the same distance behind the boat a every release.
As the cat pulls Max out of the bin Duke put him in. Max has no leash on his collar. Although in the shot before and shot after he does have a leash on his collar. The clasp of the leash is visible to the right-hand side of Max's collar (his left), meaning the leash is hanging down inside the bin itself, out of sight.
The dog-walker has gotten the dogs while their owners had left them at their apartment room by themselves, which means that he's been entrusted with spare keys.
The hot dog billboards and boxes advertise 'Weiner' rather than 'Wiener'.
When Max walks backwards into the garbage can to get away from the cats, it closes by itself.
Neither one of the Animal Control officers have baskets attached to their nets.
The cat that slashes off Max's collar is first seen with blue eyes. When the cat shows its claws, its eyes are white with small black pupils, which can also be seen as it runs to the end of the clothesline. When it reaches the end of the clothesline and cuts it, its eyes have changed back to blue eyes, even though it is using its claw to cut the clothesline.
Max and Duke should have gotten sick from eating the sausages because they did not eat them in moderation.
When Max is trying to save Duke in the end, he is walking on the windshield of the truck to get to the keys. Both sides spider and shatter underneath him. Ford started requiring all their vehicles to have laminated safety glass in 1929. It is now a standard for all vehicle windshields. This would not have splintered and shattered away underneath the dogs feet. It would have been relatively safe for him to walk across it with no issues.
The 2016 DVD has the former DVD logo from 1997 instead of the normal DVD logo that was introduced in 2001, which is on the spine and back cover.
When Max is being pulled past a city bus, the bus makes the sound of a Doppler truck horn, rather than a bus horn.
When Sweetpea turns on the TV at the beginning of the film, it uses an Xfinity remote. Comcast service is not available in New York City.
When the pets are seen on the ledge of a YouTube billboard, an Xfinity billboard can be seen. Comcast service is not available in New York City.
The only character who does not react when the Viper is killed is Ripper.
When they are asked what their method was of getting rid of their "owners", Duke and Max forget to mention to Snowball and the rest of the Flushed Pets that the cats in the alley slashed off their collars.