The issue that makes this a dubious topic is the one that makes it a possible topic; our limited perception and understanding.
...
Wait! Comeback! I'm not trying to be a jerk! I'm just saying, I don't remember a time when there was 100% unanimous predictions about the quality of a card. Let's take an example of Pidget-EX; while it wasn't part of the BDIF, I remember
most having a low opinion of it, and would never have predicted a deck built around it could take 9th place at a Regional... but that happened. So, why is it not an example?
@crystal_pidgeot recognized that it had potential, at least if I remember correctly.
Unless someone was all "Hey! Donphan has potential! All that is required is..." and predicted the many chances to the card pool and metagame that made Donphan work,
not the least of which was Pokémon Catcher receiving an erratum so that it required a coin flip to work, I'm pretty sure it shouldn't count as an example.
In fact, I wouldn't count
any time a card becomes good due to the release of a future piece of support or because it countered a future popular card
unless someone predicted that... oh wait, we're looking for cards no one saw being good, and like I said, usually one person at least claims a card will be good. @_@ If we were evaluating someone's predictions about whether or not a card was good or bad, I recommend it be about more than a "Yay"/"Nay" thing.