attackfish

When Harry was got the memory from Slughorn in six, Dumbledore still had the fake one that began with real events. I do not believe Slughorn would have turned over any memory, even to Harry if the extraction left him unable to recall the event. I believe that Dumbledore told Snape that it would so that harry could have a small chance of finding out about the best part of Snape without breaking his promise not to tell. Albus would be depending on harry's insatiable curiousity and the fact that he knew harry would know darn well what snape might be putting in the pensieve.