Audio problem: When Sayid and Essam sit in the truck with explosives, Sayid says, "Her name is Nadia, they know where I can find her," and in the shot facing Essam, when he is done with that line Sayid's mouth continues to move, but nothing is heard.
Continuity mistake: When Boone is tied up, with the goo in his hair, Locke has only his backpack slung over his left shoulder, when he holds the sheath in his left hand as he raises his right arm to throw the knife. After a brief cut to shocked Boone, Locke is in mid-throw and suddenly his second bag is slung diagonally across his chest and shoulder.
Continuity mistake: As Kate and Jack head back to the beach, while Jack explains why they should move to the caves, when Kate lifts the bag and slings it round her shoulder, opposite sides of the bag face out between the first and second shots. Then, in the third shot it hangs from her shoulder very differently.
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Continuity mistake: When Sayid goes over to Shannon to help her with her luggage her trunk falls down open and some clothes fall out. Then Sayid picks up some shirts. But when it cuts to Shannon grabbing them from him he is holding fewer and different looking shirts than he picked up first.
Revealing mistake: Season 3, episode 1, "A Tale of Two Cities.": In the scene where Ben/Henry is presenting a breakfast table to Kate, he mentions coffee which we can see is in a French press. Ben/Henry "finishes" the coffee by pushing down on the plunger of the french press, but uses only one finger with barely any pressure. In reality, a french press takes a significant amount of pressure to get the plunger from top to bottom, more than could be generated by just one finger. Additionally, we can see that the mesh-screen component of the French press stays at the top, rather than plunging down, which it should in order to filter out the grounds.
Revealing mistake: When Sayid, Kate, Sawyer and the others make camp for the night, on their way back from hearing the French woman's transmission, they sit around the fire talking. Sawyer has a cut over his right eye, but in every other close-up of Sawyer the cut is over his left eye. The shots are flipped.
Continuity mistake: When Eko finds the "priest" on the ground, his white clerical collar is properly on. In "Duex Ex Machina" when Locke causes the skeleton of the Nigerian "priest" to fall from the tree, the collar comes undone, and the clergy shirt is rumpled.
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Continuity mistake: When Jack wakes from his close encounter with Kate's sleeping pills, in the close-up his white pillow lies on the blue seat cushion, but in the next shot it lies on the grey blanket, which is spread over the cushions.
Revealing mistake: Jack's left arm is bandaged from Boone's blood transfusion. After the shot of Shannon, as she cries over Boone, Jack sits on the beach in the wideshot and the bandage is on his right arm, but in the following shots it's on his left. Yet another flipped shot.
Continuity mistake: Season 4 - The Beginning of the End - Hurley is swimming in the ocean. He sees Desmond return with the boat, and by the time he gets out of the ocean and walks over to Desmond and yells, "Where's Charlie?" his shirt is dry with sweat stains on it.
Continuity mistake: Ep. 4-12 "There's no place like home". In the first flashforward, the Oceanic representative talks to the survivors about meeting the press. As she finishes by saying, "See you on the ground", you can see baby Aaron on Kate's lap, and she is holding his left hand in hers down by his side. Cut to a close-up of Kate, and Aaron suddenly has both hands free and is holding them up to his mouth.
Continuity mistake: Episode 3-16, "One of Us". When Juliette and Ben are talking towards the end, Ben is first seen with his hands folded (fingers interlocking) in front of him. Just after Juliette says "I'll admit to it, tell her it was the only way to gain her trust", Ben's hands are in the same position, but are not folded, his right hand is just lying on top of his left. Cut to a close-up of Ben, and his hands are once again folded.