Client/Server Mismatch
I opened the new League client today (12/7/16) to get into a game. I knew there would be an update and found it strange how quickly the client opened up. I got into a game with my friends but as soon as it was loading up I received an error message saying there was a "Client / Server Mismatch", meaning my client had not been updated. I closed out of the new client and relaunched the legacy client to begin the update. Unfortunately my team had already voted to remake the game before I could reconnect.