Insert the movie, and go to chapter 6.

In this scene we see Ron's father asking to talk to Harry in private. He takes him over to the corner and has a conversation. Now, pause the movie at 17:36. Then go frame by frame until you see the number, "300" quickly appear, then disappear on the wall that Sirius Black's wanted poster is on.

This number must be a reference for the special effect. I have no idea why they didnt catch it..