Sorry DtG, I'm referring to a prior film/book. Can't recall which one, but it's where Dumbley asks Snape to give Harry lessons in protecting his mind from 'legelimens' invasion by Voldey (the reasoning being if Harry can read Voldey's thoughts then the door will work both ways, and Voldey may figure that out - as such Harry must learn to defend himself from Voldey what with Voldey being the most powerful mind invader/reader/controller of all time).

In the scene in the book, Harry enters his private lesson with Snape to witness Snape in the act of syphoning off some of his memories into the Pensieve (why is unexplained, but we already know that Dumbley often does this to help him concentrate clearly on a task at hand). Later Harry is piqued with curiosity and he snoops on Snapes memories held in the Pensieve, this is the flashback scene where he sees Snape being tormented and bullied by his father's gang of chums (thus explaining Snapes' hatred for his father).

In the film however, the flashback is provoked by Harry getting angry with Snape during his lesson and casting a Legilimens spell at Snape. He spirals into Snape's mind where the scene is revealed.

As I said above, such a rewriting of the scene makes a nonsense of Snapes role as Dumbley's secret agent working next to Voldey - his powerful ability to protect his mind from invasion being the very reason he's able to risk doing such a dangerous job. As we also know that Harry is supposed to be lazy and crap at such magic, it's a doubly nonsensical rewrite.