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: 1) Snape's motive for hiding the sword in the pond is never explained "in the movie." As for the book, it explicitly states that Dumbledore instructed Snape to give the sword to Harry when possible, but make it so it takes bravery and courage to acquire, like a true Gryffindor. Snape then found they were in the Forest of Dean, and he himself placed the sword there. 2) The Locket was partially sentient and could sense that the sword was a danger to it.
raywest ★
If he placed it somewhere anyone could access, the sword might get into the wrong hands.