Question

how come I just got a master ball when I first met professor juniper at mistralton city in white 2

Answer 1

This is the special master ball that you are guaranteed to get in the game

Comments

Not exactly. You can get 999 Master Ball by trading Pokemon that hold them. This one is just part of the story.

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That's what I ment by "guaranteed"

Answer 2

Because GameFreak wanted it like that. Not really much to say.

Comments

Lol, I used it right away on a minccino

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Ewww...

Answer 3

No one knows why. Save it for later, or use it off. You get one later anyway by defeating Corless post-game. Maybe for the newbies who find it difficult to catch a legendary.