Quote Originally Posted by Peter_20 View Post
What's this?
GameFAQs is your friend.

Info from here

The treasure chests' contents do not depend on their location, but how many you have opened. If you touch a Save Sphere, the counter will reset.

1) 1x Lv.4 Sphere
2) Defending Bracer (2 slots) Silenceproof, Darkproof
3) Turnover (Magic Counter, Counter-Attack)
4) 2x Lv.3 Key Sphere
5) Defending Armlet (Stoneproof, Poisonproof, 2 FS)
6) 2x Friend Sphere
7) 1x Lv.4 Key Sphere
8) Phantom Ring (Ice Eater, Fire Eater, Lightning Eater, 1 FS)
9) Cactuar Wizard (Half MP Cost)
10) Warmonger (Double AP, Double Overdrive)
11) 2x Teleport Sphere
12) 99x WARP SPHERE!

Do the chests disappear for ever, or do they come back after a while, or do I have to open them correctly on the first try?
Yes, no, and you don't have to, but if you want 99 Warp Spheres, you should try.

Do the chests' contents randomly change every time you reload the file?
Yes.

Are there miss-able items in there?
No, the chests are empty. :rolleyes2 You don't need them any more than you need every non-story related item, though.