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: Because Harry already knew where the camp was. The protection spells they use (Harry is shown at one point casting them as well) seem to be sort of like the Secret Keeper spell. If you cast them or they are cast on you, it allows you to see through the spells. Also, don't forget that the deluminator led Ron right to the general vicinity of the camp and that the Snatcher team could smell Hermione's perfume through the spells earlier.
Guy