(For example, another effect might cause that ability to trigger an additional time.) In such a case, even if you pay mana as the triggered ability resolves, this permanent won't transform because it already transformed while the triggered ability was on the stack
Its a really strong card
You won't sacrifice anything, and there's no penalty for being unable to do so
According to WotC's official article describing Mystery Booster 2's contents, you'll open a traditional foil Alchemy card in less than 1% of boosters , always replacing the Future Sight card slot
By all accounts, though, they have more-or-less picked off right where the Magic Invitational winner cards left off
Once we got a better sense of how scarce those cards actually were, the price dropped