Thalia School of Wizard bug
I had school of wizard complete. My opponent played a Thalia. My next turn, my Second Sun was now costing 8 instead of 6 as expected. I played the second sun, however I did not get my water elemental for playing 6 mana worth of spells and then died on this attack. I should have had 2 elementals to block and keep me alive.
To add to this, in a different match, I played the 2 mana owl from School of the Wizard and reduced my Second Sun's Dawn to 5 mana. Later I played the 5 mana Second Sun's Dawn and no other spells, but got my elemental.