During a Premier Draft, my opponent removed my defenders and attacked for lethal damage. I had two Kill Shots in hand, with enough untapped mana to cast both and destroy two of the attackers and prolong the match.

I click-and-dragged one of the Kill Shot cards onto the battlefield in order to cast it as soon as I got priority after the opponent declared attackers, but I never had the chance. Instead, the client immediately passed priority and damage occurred, and I lost the game. That was my 3rd loss, ending that draft.