Every time Locke's envelope is shown, the handwriting is different. Also, sometimes Jack's name is in pencil and sometimes it's in ink.
At the start of the episode Jack wakes Kate up on the Island. Kate asks if they are back. When the scene is replayed later in the episode, when time catches up to the present time, Jack wakes Kate, though she does not ask if they are back.
During the Flight 316 crash scene the plane makes noises like an empty barrel, as if fuselage panels were banging against each other. Even in severe turbulence airplanes don't make such sounds. Usually it's just plastic panels squeaking and rattling.
The cockpit scene shows three thrust levers even though the Boeing 737 only has two engines.
On the flight to Guam there is a lighted sign in the aircraft that says "No Smoking / Fasten Seat Belt / Use Seat Bottom Cushion for Flotation". Transoceanic flights are required to have life rafts and life jackets for every passenger. "Seat Bottom Cushion" equipped aircraft are restricted to overland flights and those that stay within just a few miles of the coastline.
During the flight back to the island, Jack is nervous and asks Ben "How can you read?" Ben calmly responds "My mother taught me." Ben's mother died during Ben's birth. This is not an error. Ben was merely being flippant with Jack.
In the beginning, Jack dives from a very high height - maybe 50' or so - into the lagoon, then finds it to be shallow enough to stand up in. In reality, diving from that high a height into shallow water would likely result in a broken neck - the water would not be deep enough to cushion the impact. However, Jack swims away from where he dove in at least 15 feet. Then he contacted Hurley and swam with him further away. Where they could stand was quite a bit away from where he dove in.
Eloise makes a big show of turning a wheel in the center of the door to the pendulum room. But on the other side of the door the fastening mechanism is a simple dog with no need for a wheel.