i assume desmond was the only person who could remove the magic cork, as he had to wade through and the magic pee water. when jack was in the cave, there was no magic pee water to speak of.
But once Jack put the cork back in, the glowing magic urine started back up. So jack should have died at that moment, not climbed out and died in the forest.
And how did Des get off the island at the end of the series? He wasn't on the plane and they kept telling us there was no other way to get off at that point.
