I got a bad opening hand playing as a Priest against a Mage, and made the near fatal mistake of trying to use Crazed Alchemist on my opponent's Mana Worm the opening turn. I though, "It's only got 1 health now, how long can it last?" Long enough to grow to an 8/1 and take my health down to the teens. It was a rough battle, and I was sure I was going down. Luckily the guy used a polymorph on my Northshire Cleric, and had to leave the sheep alive (which is what killed his 8/1 worm). I think that is the mistake that cost him the game, since my Ragnaros was what killed him. I also got lucky and pulled a Flamestrike out of his deck with my Thoughtsteal.
I did miss a really good possibility, though. He played out an Ogre Magi 4/4, Sen'jin Sheildmasta 3/5 and a Mana Worm 1/3. I had Ragnaros on the board, and my hand consisted of Temple Enforcer, Abusive Sargent, Holy Nova, and a couple other cards that I don't think mattered. I should have used Holy Nova first, so that I didn't end up healing the Sheildmasta while it was on my side, but that ended up not mattering because Ragnaros went for my opponent's face and killed him. Ended up being a far riskier move than it should have been.