Factual error: When Marley runs on the field during the Marlins' Bark at the Park night, the stadium scoreboard can be seen briefly behind him and John. "WAS" (Washington) is listed as one of the teams playing a game that night, despite the fact that Washington did not have a team before 2004 and the film takes place in the early 90s.
Revealing mistake: When Marley is running away and jumps the fence, John follows and lands on a crashmat disguised as a flowerbed. If you watch closely there is movement in areas John is not by, and this movement is similar to that of a corner of a crash mat going up when someone lands on it.
Continuity mistake: Just before Marley jumps out the car, he has his two front legs outside the window. Camera cuts to a wide shot from the pavement and his front legs are in the car. Back inside the car, his legs are outside again. This is not to be confused with a similar mistake where he is walking with his back legs in/out the car. (00:26:00)
Continuity mistake: In the scene where John takes Jenny to pick out Marley, Jenny has a tie on as a blindfold. When they first get out of the car, the tie ends are hanging long and straight down her back, but when they get on the porch, the knot has changed and is sticking out the side of her head.
Continuity mistake: When John and Sebastian step out of the elevator discussing Jen, the spare room, step seven, etc, John's bag strap hangs across his chest with the bag at his right, in the shots facing them once they reach the newsroom, but in shots from behind John's bag hangs straight down on his shoulder with the bag to his left. (00:08:25)
Continuity mistake: When the dog-sitter pulls up in front of the Grogan's house in the exterior shot, the curtains at the window beside the stairs are pulled nearly closed, with only a narrow gap between. Next interior shot, as Marley barks at the window the curtains are pulled more apart, but when Marley pulls Debby outside as the Grogans leave, the curtains are as they were in the first exterior shot. (00:44:10)