Continuity mistake: Near the end, when Hermione jinxes Harry's face to disguise him, she removes his trademark glasses to hide his identity and shoves them into her own pocket. Harry is still without his glasses during the confrontation at Malfoy Manor where his identity remains in doubt (his glasses would give him away). However, as Wormtail (Pettigrew) is taking both Ron and Harry to the cellar, Harry's glasses are suddenly back on his face, even though Hermione was separated from the group.
Continuity mistake: After Ron destroys the Horcrux, Harry goes to sit beside him. The distance between him and Ron increases in the next shot.
Continuity mistake: When Mundungus is confronted about the locket, as he says, "Could I help it if Mad-Eye fell off his broom," Ron places his hand on the chair with his hand facing away from Mundungus. Depending on the angle of the following shots, Ron's hand is either facing Mundungus or facing the opposite way.
Continuity mistake: When Hermione apparates herself, Harry, and Ron away from the wedding, she does not have her beaded bag, nor is it on her person when they first appear in London. After ducking into the alley, she suddenly has it in her hand. From then on, she always has it.
Continuity mistake: After Ron pulls Harry out of the hole in the icy lake and leans Harry up against a rock, we see Ron back up a few feet. When it cuts to Harry getting up, Ron is much further away.
Continuity mistake: When Harry dives through the hole in the icy lake, water floods over the edge of the hole, and flows over a large part of the ice, dampening the snow on the ice. When it cuts to Ron pulling Harry out of the hole, the wet snow around the hole has decreased by several feet, only surrounding a small part of the hole itself.
Continuity mistake: In the first shot of Harry, Ron, and Hermione at the café after escaping the wedding, Harry asks if they should go back. Then the waitress walks past the camera. When it cuts, the waitress walks past the same spot again.
Continuity mistake: When Hermione picks up the "Albus Dumbledore" book in Bathilda's house, she picks it up with her left hand, while her right hand is holding her wand only. When it cuts she is holding the book with both hands.
Continuity mistake: When Harry holds the mysterious locket up in front of Kreacher, the chains are together from Harry's POV and separated from Kreacher's.
Continuity mistake: When Mundungus arrives at the house with Kreacher and Dobby all over him, the pot, glassware, wire basket, bottle, etc. on the counter beside the sink disappear, reappear and/or change entirely in the close-up and following shots.
Continuity mistake: When Harry is holding his wand over the icy lake, the light is reflected on the surface. When it cuts to a closeup the reflection is all gone.
Continuity mistake: When Harry, Hermione and Ron confront Mundungus about the locket, he sits down in the corner chair and the side table is beside the chair, with the mess of Daily Prophets on the floor in front of the table. However, in the following shots the table is 3-4 feet away from the chair, against the wall, with one Daily Prophet between it and the chair.
Continuity mistake: In the diner, when both Hermione and Ron aim their wands at Dolohov, the boxes of Amaretti cookies explode off the shelf behind him. However, in the following shot one Amaretti box (facing the wall) is still on the shelf, then vanishes and reappears once again.
Continuity mistake: In the scene in the Cappuccino Cafe, Hermione performs "Petrificus Totalus" on a Death Eater and Ron talks about killing him. Harry says, "It's best we wipe their memories." As he says that, he is looking down at the Death Eater. In the very next shot, his head has slightly moved and he is looking straight at Ron.
Continuity mistake: On the beach when Harry is holding Dobby, and Dobby says "Dobby is happy to be with his friend", Harry is holding Dobby by his shoulder. In the next shot Harry is holding Dobby by his neck.
Continuity mistake: When Ron, Harry, and Hermione are sleeping at HQ, the camera shows Hermione and Ron sleeping on the couches, laying opposite ways, with their hands almost touching. The next shot is of Harry waking up, and now Ron is sleeping on the floor and Harry is on the couch.
Continuity mistake: When Harry is in the water going after the Sword of Gryffindor he's wearing his glasses; but after Ron pulls him out of the first thing he does is reach for his glasses on the shore.
Continuity mistake: When Bellatrix looks at Harry through the gate at Malfoy Manor, her lips are a pink color. Inside the house, when she shows him to Draco, her lips are a totally different dark red shade.
Continuity mistake: In the beginning, when Minister Scrimgeour is making a speech, there are some people standing behind him. When he finishes and is being photographed, there are suddenly no people behind him.
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