This takes place shortly before Harry and Dumbledore return to Hogwarts from seeking the Horcrux in a sea cave - it is the prelude to the invasion by Death Eaters.
To me this is (by a margin) the best scene in any of the Harry Potter movies (so far); in particular for the marvellously powerful yet understated acting of Alan Rickman as Snape. If you know the back-story you will recognize that this is first-rate stuff.
Yet it ended on the cutting room floor; only being released as a cut-scene 'extra' in the 2 volume DVD.
I sometimes wonder about the competence of movie editors.
Particularly as the HBP movie was padded-out with dull 'action' sequences; and inexplicably, amazingly, failed to include the vital piece of information as to why Snape had called himself The Half Blood Prince - thereby failing to explain the movie's title.