I think there is much more to Jack's story than we know.
In the pilot episode, he is the only person to have landed in the Jungle whilst the rest of are survivors are on the beach. Many people have speculated that he was flashed to the Jungle in the Pilot episode, similar to how he was in Season 5. Although we can clearly see jack is wounded and bruised from his fall and that he probably wasn't flashed there in the Pilot episode, it is still very strange that something has made him land there, in the jungle. Add to that the strange mysterious black object that is lying next to him, too. It makes me feel as if during the crash of 815, something has helped him land, not through flashing, but some extra force has made him end up in that exact spot, for some unknown reason.
Could it be that he has been to the island before, and he can't remember it? Or can he remember, and he is just not letting on?
And if he hasn't been to the island before, and that is a silly theory, then just why is he in the jungle, lying next to a strange black object in the pilot episode, whilst all the other survivors ended up on the beach?
I think at some point in Series 6 there will be a huge revelation about Jack's past involvement with the island...