Deliberate mistake: When Boo laughs repeatedly while playing hide-n-seek with Sully in the men's room, nothing happens to the electricity - no surges as with all other times she laughs. [In the audio commentary by the directors, they explained that it would've been distracting if an energy surge occurred every time Boo laughed, so they simply didn't address the issue. A reasonable course of action, but still a mistake.] (00:37:30)

Monsters, Inc. (2001)
Plot summary
Directed by: David Silverman, Lee Unkrich, Pete Docter
Starring: Billy Crystal, Bob Peterson, Frank Oz, James Coburn, Jennifer Tilly, John Goodman, John Ratzenberger, Mary Gibbs, Steve Buscemi
You know when your were little, and you thought monsters came through your closet and scared you? Well, don't take it personally. It was just a job. Monsters depend on the screams of human children to power their city, Monstrapolis. So they climb through the closet doors of kids around the world, and store the screams at Monsters, Inc. This is dangerous work, as every little monster knows that nothing is more toxic than a human child - "a single touch could kill you!" So when the city's top scarer Sully and his friend Mike accidently let in a human child named 'Boo', they'll do anything to get her back into the world she came from.
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Answer: He's not as much surprised as he is pleased to see her. She's going to be a little older than when he last saw her because it took Mike so long to rebuild the door. So if he's surprised, it may be at how much she's grown in that time period.
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