When Boone is drowning in the water, Charlie claims he can't swim and calls to Jack for help. Later, in the season 3 finale, Charlie recounts that he was a school swimming champion and makes a rather lengthy swim into the underwater DHARMA station, The Looking Glass.
Right after Locke goes to search for water, the camera cuts to Jack running through the woods after he sees his father. Jack has two pairs of cuts on his face from the crash: one on his left temple, the other on his right cheek. During this sequence, there is a shot where Jack turns to reveal cuts on his right temple, a minor flip in the editing process.
When Charlie and Hurley were asking Jack what to do about the low water supply, the camera spans on Jack while under the hut and shows two people -- a man and a woman -- walk by. When the camera spans back to Charlie and Hurley, then back to Jack, the couple walk by again in the same direction.
At one point you can see Jack standing near a fire, and he hears somebody walk behind him. He grabs a piece of wood from the fire and goes to follow that person. In later shots you can see fire "dripping" from the wood, just like from a real torch, thus indicating that it was lit with oil - something he couldn't have found in the woods.