Continuity mistake: Season 5, episode 2 "The Lie": When Jack is walking into the medicine cupboard, he has a few small sweat stains on the back of his shirt. The camera cuts to show him turning to a different shelf, and for a split second you can see that his shirt back is now covered in a huge stain, and with large sweat rings under his arms. After he has injected Sayid, his shirt is completely dry again.
Continuity mistake: At the end, when we see Ben lighting a candle with a matchstick in the cathedral during the entering of Eloise, he puts out the matchstick so we see that the flame is gone. In the next shot, the flame is back and Ben is doing exactly the same thing, but we see it from another view.