Harry stops by Professor Slughorn's classroom as his class is letting out.
Harry's strategy for getting the intel? He basically tries to stage that whole memory all over again, with Harry playing the Tom Riddle part: he says he was in the restricted section, blah blah—you get the gist.
He asks Slughorn if there are types of magic that they're prohibited from teaching the children. Slughorn tries to blow him off, but Harry persists, using a little good old-fashioned flattery. But Slughorn still gives him a canned answer about trying to live within the light and staying away from Dark magic. Still not what Harry's looking for.
So he pulls out the big guns and plays a little dirty: he asks Slughorn if he answered the same way when Tom Riddle asked the same questions.
Record scratch. Oops, Slughorn gets mad, since he realizes that Dumbledore was putting Harry up to all this all along.
Later on, when Harry knocks on Slughorn's door, he gets brusquely turned away. Not looking great for that whole memory mission.