Factual error: The version of Brixton underground station is very lazily reproduced: the 'U' and 'D' of the 'UNDERGROUND' roundel have been the same size as the rest of the word since before Brixton station was built; the white circle is not part of the roundel; the station appears to be accessed through a plate-glass window; there is no reason for a black cab to be parked at the side of the road; also, the cars are all parked along the right-hand side of the road, which they would never be in the UK.
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.
Factual error: Hurley arrives at the departure gate and calls to the woman to wait and let him board. As he goes through the gate, a sign with the word 'authorized' is visible to the left. This scene supposedly takes place in an Australian airport, but this is the American spelling - in Australia, the word is spelled 'authorised'.
Factual error: In the flashback that reveals the phone call Michael makes to his mother about Walt, Walt is seen playing a Nintendo Gameboy Advance SP before the phone call. After the phone call, when Walt stands behind Michael, Walt tells Michael that he needs new batteries. However, The Gameboy Advance SP works with a rechargeable battery which cannot be replaced with common batteries.
Factual error: The prison Desmond is released from looks nothing like any British prison, let alone the British military prison (the "Glasshouse"), which is more of a training camp than a traditional prison. The guards wear no cap badges on their berets (all British service personnel wear cap badges) and their berets are worn almost vertically on their heads, whereas British berets are pulled hard to the right. The NCO who releases him is credited as a "master sergeant" in the episode credits - an American rank which has never been used in the British Army.
Factual error: Season 6, episode 9 "Ab Aeterno": The episode is supposed to take place in the Canary Islands, Spain. However all the characters speak with either Mexican, Puerto Rican, or Cuban accents. In the 19th century there wasn't such a blend of American cultures.
Factual error: When Charlie is in the small communications room of the underwater base, after the grenade blows out the window and the water is rushing in. The water would only come up to the top of the window, anything above that would be an air pocket and water would not go any higher, yet it got completely filled over Charlie's head.
Factual error: Naomi Doret, the woman who parachuted onto the island, is explaining about the outside world finding the wreckage of flight 815 "off the coast of Bali, in an ocean trench 4 miles deep". The problem with that statement is that Bali is nowhere near the Pacific Ocean, it's in the Indian Ocean. Oceanic flight 815 was a direct flight from Sydney, Australia to Los Angeles, US. That flight wouldn't go anywhere near the Indian Ocean or Bali.
Factual error: This is across the whole show, but made perfectly clear in the episode "The Whole Truth" (Season 2). Korean women do not change their last names upon marriage. A woman in Korea cannot be identified by name as someone's wife. And they would never be addressed that way. But Sun's fertility doctor addresses her as "Mrs. Kwon". Korean women can adopt their husband's family name if they move to a country where that is commonplace, but Sun was still in Korea at this point, and even after they crashed on the island, she had only just left the country.