Very interesting! [Possible spoilers]
So I'm on the third disc, right, and I'm up to the Northern Crater, except I want this run through to be the one where I complete everything, etc.
Anyhoo, I'm just cruising round, and decide to go to Nibelheim. So I go to the mansion there, and at the bottom, as soon as I walk into the lab, this kind of flashback scene happens. It goes for about 10 minutes, and it shows some lab assistant in the basement there, and there are 2 bodies floating in these mako tubes. One of them opens and kicks the assistant to the floor, and lo-and-behold, it's Zack! Zack the opens the other Mako tube, and out crawls Cloud, obviously delirious from all the mako.
It then fast forwards to Zack and Cloud being in a ute, which Zack's pop is driving. They're on their way to Midgar, and Zack has a fair bit of talk with Cloud. He discusses what he wants to do, and ends up saying he wants to be a mercenary, and he wants Cloud to be one as well.
It fast forwards a bit more, and Zack is pulling Cloud up a kind of hill thing. Cloud is still infused with Mako. You then hear gunfire, and Zack goes off screen, there is a bit of sword clashing, and then Zack comes back. He kneels next to Cloud, then bullets fire and Zack gets shot back. 3 shinra walk onto the scene, and shoot Zack a bit more. They ask about Cloud and the general just says to leave him. Cloud then slowly crawls to Zack's body, stays there for a second, then (by looking at his body), grabs the sword and opens his arms, in the air, as if screaming "WHY". Then it ends.
Now, I don't know if anyone else knows about this, but I've never heard it mentioned before. It's very interesting and reveals more about Zack. It also shows how Cloud came to be a Mercenary.
Your thoughts?