X-Men: The Last Stand
The villain Magneto has started to put his Brotherhood of Mutants together.  While freeing his right-hand man Mystique from jail, a guard fires a "cure dart" at him to try to remove his mutation.  Mystique jumps in the way and is cured of her mutation.  Magneto leaves her.  As a result, she tells the government where Magneto will be.  The Brotherhood meets in a forest.  Wolverine, who has gone out there, sees that the Phoenix has teamed up with the Brotherhood and will not leave when Wolverine asks her to.  The government then raids the forest, but Magneto has already left and all the agents find is many copies of the mutant Multiple Man.