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 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: 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.
Chosen answer: It's explained a little more in the book. Hermione swiped Mad-Eye Moody's stock of the potion when Mrs. Weasley wasn't looking. And it's possible that, when they ran low, she simply replicated it with a Doubling Charm.
Cubs Fan ★