Audio problem: When Scott fights Ramona's ex-girlfriend, Ramona holds his hands while punching her. One punch misses her but the sound of a punch is heard.
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More mistakes in Scott Pilgrim vs. the World
Young Neil: He punched the highlights out of her hair!
Trivia: The scene when Ramona delivers the package to Scott and he signs for it, he takes the package and throws it over his shoulder into the trash can, some several feet behind him. In the outtakes reel, they show that no special effects were used. Michael Cera made the shot over his shoulder on his 33rd attempt.