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)
1 deliberate mistake - chronological order
Directed by: Pete Docter, Lee Unkrich, David Silverman
Starring: John Goodman, Steve Buscemi, Frank Oz, Billy Crystal, James Coburn, Jennifer Tilly, John Ratzenberger, Bob Peterson, Mary Gibbs

Other mistake: Near the end of the film, when the identity of 0001 of the CDA is revealed, number 0112 is shown both standing next to 0001 as Roz enters the room and behind Sully and Mike on the opposite side of the room. (01:16:45)
Henry J. Waternoose: There's nothing more toxic or deadly than a human child. A single touch could kill you. Leave a door open, and one can walk right into this factory; right into the monster world.
Trainee: I won't go into a kid's room. You can't make me.
Henry J. Waternoose: You're going in there, because we need this. Our city is counting on you to collect those screams. Without scream, we have no power. Yes, it's dangerous work, and that's why I need you to be at your best. I need scarers who are confident, tenacious, tough, intimidating. I need scarers like... like... James P. Sullivan.






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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