This is taken from an article on a german mmo-site, link underneath.
Note that this is rather loosely translated and may contain errors, as i don't play Hearthstone myself and have no idea about it's mechanics, decks etc. I just found this interesting in general.
After a power failure corrupted a database in China, Blizzard wasn't able to restore the latest state of it and had to rollback for four days.
Players affected by the loss of 4 days gaming time got compensated by Blizzard with 5 classic packs, 5x Whispers of the old gods, 5 x Gadgetzan and 1000 gold.
Due to the rollback, players were able to compare the content of packages before and after and in which way the cards may possibly vary, as assumption was that there is a guarantee in place that grants you a card of X rarity within a certain amount of packs.
If a player found a rare card in his opened 5th pack before the rollback, he also found a rare card in the 5th pack after the rollback. The card itself may have been different, but the rarity level was always the same.
That means that while the cards are being taken from a pool of cards and rolled out by RNG, the rarity is predetermined.
http://mein-mmo.de/hearthstone-china-datenverlust-entschaedigung/