In the game, never. At least that's widely known though.

The first time I asked for help with Pandora's Box on this forum, nobody seemed to know about the first Dragon Zombie rule. Eventually I turned to the E-Skill faq on Gamefaqs.com and it listed it as a rumor. Up until then I thought it was with my game only.

A lot of people have disputed the first Dragon Zombie rule, so I've assumed it's a glitch if it's not universal to all versions of FFVII.

So I take it you've found the missable stuff in FFVII frustrating as well?