Continuity mistake: Season 3, episode 2, "The Glass Ballerina". When Jin is in the hotel and is about to kill Jae Lee, he holds a pillow to Lee's head and presses his gun against it. At first, the gun is pointing to Lee's cheekbone, but when it cuts to Lee as he is sobbing "I'm sorry. I'm sorry", the gun is much lower, and pointing to his throat. Cut back to Jin, and you can see the gun is once again pointing to Lee's face. There was no time for Jin to move it between shots, and Lee did not move nearly enough to cause this changes.
Continuity mistake: When Sawyer helps push the minivan, there are small holes on his tee shirt's collar and a large hole on the seam of his left sleeve. When he walks back into camp with the beer, the large hole is now on his right sleeve, and the left is intact. Neither are flipped shots.
Factual error: In the first flashback, when Charlie first hears himself on the radio, there is a close-up of the UK number-plate on the front of the van. It is the wrong colour: this is a yellow plate. As it is on the front of the van, it should be white. Only rear plates in the UK are yellow. This false plate was made up in mirror-image by the production company, as the actual van used was Left-Hand Drive. The whole scene, apart from the shot of the radio, was filmed and then flipped for the final cut, to make the van appear to be Right-Hand Drive as you'd find in the UK. Hence the moles on the right of Charlie's neck are on his left in this scene.
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.
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.
Continuity mistake: When the entire camp is heading up to the radio tower they head up the beach with the water on their right side. After the title screen shows they are walking with the water on their left side. They are going in the opposite direction as before. (00:09:10)
Continuity mistake: When John loses his voice, he uses a pad to communicate with Charlie. He writes "I need you to stand guard" in one scene and later shows the same message to Charlie. In the second scene, although it is supposed to be the same message, guard is misspelled "gaurd."