Good ol' broken Mad Scientist.
If you use Fire Elemental on Mad Scientist, it pulls out Mirror Entity and then puts a Fire Elemental into play!
Of course the last time Blizzard tried to "fix" this by removing death processing in certain phases, they introduced other problems. The problem ultimately isn't mad scientist. It's that secrets are coded so that they are lit up and await triggers immediately upon entering play, regardless of any other actions in the queue. Right now we see this through the deathrattle of mad scientist, but if there were other ways that you could force your opponent to play a secret, it would have the same problem. For example, what if Blizzard introduced a minion that had a battlecry that gave both players a secret? Same issue. That would trigger mirror entity.
The "fix" should be that if a secret enters play, it should initially start in a darkened phase where it does not accept triggers. Then, the last action in the queue should be to light it up and begin to accept triggers. So Mad Scientist dies, the secret enters play, it starts out dark, the phases resolve, and then the secret turns on and lights up. That way, you always have the choice of when to try triggering the secret.
