While playing as a Warrior/Priest in Arena I discovered the following bug.
Turn 1: I play Athletic Studies - I discover Imprisoned Vilefiend
Turn 2: I play the Imprisoned Vilefiend that I just discovered for the discounted 1 mana. I leave 1 mana floating.
Turn 3: I notice that the Restless Mummy (which I drew from my deck on turn 1 or 2) in my hand still costs 1 mana less. It should cost 4 mana but it costed only 3 mana. I decide to play it. Because of the error I win the game.
Appearently the game didn’t realise that I used the Athletic Studies discount on the Imprisoned Vilefiend because it started dormant.
I would include a link but this website doesn’t allow me.