Revealing mistake: When the Order members drink the polyjuice potion, Hagrid is talking to Harry but he is not visible in the mirror on the wall. (00:14:50)
Revealing mistake: Throughout almost the whole movie Harry's glasses don't have lenses in them. Only in two or so scenes do they appear.
Revealing mistake: Watch as Ron pulls Harry out of the icy lake and leans Harry up against a large root. When Harry's back comes in contact with the root, it wobbles. Unless Harry weighs a couple of times more than he does, the root wouldn't flinch.
Revealing mistake: When Harry walks across the frozen pond, both in his shoes and when he is barefoot, there are no footprints left on the ice. The pond is covered with a light snow, that Harry brushes away to see the Sword of Gryffindor beneath the frozen surface. The snow would show any tracks.
Revealing mistake: When Harry uses the spell "Engorgio" on the fire in a jar while talking to Ron, there is a jump cut as the flame gets tall.
Answer: Lucius and Bellatrix were unable to confirm Harry's identity because Hermione had jinxed his face to disguise him just before the snatchers captured them. They are fairly certain it is Harry, but as Bellatrix says, if they summon Lord Voldemort, and they are wrong, he will kill them all. Of all the Death Eaters at Malfoy Hall, no one knows Harry better than Draco, having spent more time with him because they were schoolmates. Lucius and Bellatrix have to be absolutely sure that this is Harry Potter.
raywest ★