Revealing mistake: When Goodwin and Ana Lucia sits down to have a break and Goodwin hands over some fruit to Ana, she says "Where do you think they got it? I mean they're not even wearing shoes", directing to the knife. If you watch in the background you can see a car driving.
Revealing mistake: After Sawyer has been shot with the syringe thing (I'm not sure if Kate had been shot yet) Jack fires his gun five or six times into the jungle. The last few times he fires it there is no flash or any indication that the gun has been fired.
Revealing mistake: As several camera angles show, Sawyer's makeshift stretcher was made by drilling multiple holes in the upper frame struts, through which was threaded rope made from dried roots or vines. The cast who supposedly constructed it were not carrying power drills among their primitive implements.
Revealing mistake: In Season 2, Episode 18 - Dave: When Ana Lucia is threatening "Henry Gale" in the hatch cell; every time he moves and his makeshift shirt slips, you can see a line on his right upper arm where they applied make-up. Apparently he's wearing makeup on all the exposed parts of his face and neck to make him look sunburned, exposed to the elements etc. This is not just a normal suntan line, as his shirt wouldn't normally fall down that much out in the open and he's been inside for days now.