Yes don't worry, it won't be charged if it's an ERROR.. I got it like 3 times trying to subscribe to Arena Tournament, my CC called me, they said the 3 transaction is approved, but not yet charged.. I asked in the Blizzard forum and they tell me it won't be charged and will be returned back to me if the transaction didn't not SUCCEED/CHARGED. After 1 month, the transaction didn't appear on my bill..

So don't worry, it won't be charged if it's an ERROR