I believe there is a small mistake in the movie script of Harry Potter and the Prisoner of Azkaban.
The error is when Professor Lupin says “Mischief managed” to the Marauder’s Map, just after saying goodbye to Harry at the end of the movie.
In the book, Harry finds Lupin packing in his office, and Lupin says, “I saw you coming,” pointing to the map. On the next page, Lupin says to Harry:
“I am no longer your teacher, so I don’t feel guilty about giving you back this as well. It’s no use to me and I daresay you, Ron and Hermione
find uses for it.” (PoA pg. 424/309)
But he doesn’t work the map in Harry’s presence.
However, in the movie, at the end of this scene, Lupin points his wand at the Marauder’s Map and says, “Mischief managed.”
Very cute way to wrap up the scene. But there’s a problem. We know that Fred and George stole the map from Filch:
We let off a Dungbomb in the corridor and it upset him for some reason. So he hauled us off to his office and started threatening us with the usual, detention, disembowelment, and we couldn’t help noticing a drawer in one of his filing cabinets marked Confiscated and Highly Dangerous.” … “George caused a diversion by dropping another Dungbomb. I whipped the drawer open and grabbed this. It’s not as bad as it sounds, you know,” said George. “We don’t reckon Filch ever found out how to work it. He probably suspected what it was though, or he wouldn’t have confiscated it.” “And you know how to work it?” “Oh yes,” said Fred, smirking. “This little beauty’s taught us more than all the teachers in this school.” (PoA pg. 191/143)
Fred and George had no connection with the original marauders, James, Sirius, Lupin and Peter Pettigrew. So, Fred and George would have had no way of knowing the original incantations used to operate the map.
I believe it’s only logical to assume that Fred and George must have figured out how to use the map by trial and error. However, it’s very unlikely they would have come up with the exact incantations the marauders used, instead, it’s more likely that they came up with something close to the original incantations, and the map understood their rule-breaking intentions.
Now, Lupin, being one of the original marauders, knows the “real”, original incantations that work the map. In addition, he would have no way of knowing what Fred and George’s versions of the incantations were (and hence the versions Harry uses because Fred and George told Harry), because Harry never uses the map in front of Lupin.
“I solemnly swear I’m up to no good” and “Mischief managed” are the incantations that Fred and George made up. So, it doesn’t make logical sense for Lupin to say “Mischief managed” to the map at the end of the movie!