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.
Factual error: As John is dropping his wife off at the airport before picking up Marley, watch the background - a very modern car, possibly a Chrysler 300, is seen driving past, despite the scene taking place in the early nineties.