The End (1) - S6-E17
Revealing mistake: When Desmond moves the rock, Smoke-Locke and Jack fight outside the cave while an earthquake shatters everything. Watch the leaves around and you'll notice that they don't move at all, giving away the camera effect. This also happens later on when Hugo, Ben and Jack are about to fix the cave of light.
The End (1) - S6-E17
Audio problem: When Desmond, Locke and Jack are standing outside the cave with light, Desmond says "maybe I can find a way out for you too," or something like that. Jack replies, "there are no do-overs." The camera then goes to over Jack's right shoulder as he says, "I tried that once." However, the words don't match what his mouth is saying.
Answer: He was eating a peice of an orange with the skin still on.
XIII