In the Hearthstone tutorial, Millhouse Manastorm taunts you with what he will do once he reaches 10 mana. What does he actually do if you slack off and let him reach full mana?
I actually tried this when the multi-region support came out. He can't.
During the first two tutorials (Hogger and Millhouse), there are a lot of limits on what you can't do. Not only can you not fireball a friendly or yourself, you can't even end your turn unless you have attacked with everything possible and cast everything you can.
These restrictions make it impossible not to kill Millhouse by around turn 6 so he never reaches 10 mana.
I'd like to rescind my previous answer, as I've done more research into this topic for another question. This line of Millhouse Manastorm's is a reference to the old Millhouse Manastorm card, which had "Battlecry: Put a 'Mega-Blast' card in your hand." Megablast is a 10 mana card that deals 5 damage to all enemies.