I'm starting to think that Backstab is in that special category with Fiery War Axe where they are guaranteed to be in your opponents starting hand.
Certainties in Hearthstone:
Warrior: Will always have Fiery War Axe if you think for an instant "I hope they don't have fiery war axe"
Mage: As per Warrior. "I hope they don't have frost bo- And there it is"
Paladin: Will always have True Silver on turn 4, but only if there is a minion of at least 3 health on the board.
Hunter: If you put yourself in a position to lose to kill command, you will lose immediately to kill command.
Warlock: If you play soul fire with a Doom Guard in your hand it will always be discarded, unless you have Jaraxxus which will be discarded instead.
Druid: Playing anything with a good deathrattle will immediately draw them keeper of the grove.
Priest: Priest will always have power word shield to play on an early minion. That minion will always die in the next turn anyway.
Rogue: Playing any 3 or 4 health minion grants them an automatic backstab plus agent.