Deliberate mistake: The (fake) confession letter was postmarked from Ohio and sent to Allison Connor, 331 Henry Lane in Hilliard, MB 21243. No U.S. state has a postal abbreviation of "MB." The way the "B" was written in thick bold letters made it resemble a "D" - "MD" represents Maryland (where the film apparently takes place). Also, there is no current 21243 zip code in the U.S. There is a 21241 for Baltimore City and the next consecutive zip code is 21244 in Baltimore County. The created address LOOKS real. (00:50:44)
Deliberate mistake: All Minor Hockey Leagues in North America require players to wear visors with their helmets.
Deliberate mistake: In the scene where Marmaduke runs over Don Towmbly, you can "see" the soft mat where he lands on his face: it wobbles.