Revealing mistake: In the scene just after Sawyer is mounting the radar on the mast of the shop, Sayid hands Michael a flare gun. He opens it to show that there is just 1 shot left. If you look closely the flare gun shell has already been fired (notice the center fire pin is indented).
Continuity mistake: In the scene just after John Locke's teenage mother runs from the hospital room, the nurse converses with the grandmother. You can see her put a cigarette to her mouth with visible lipstick stains on the wrong end of the cigarette, possibly from previous takes.