Syr Konrad + Mindcrank break the game
Had this happen twice now; I thought it was a fluke the first time, but I was able to replicate it immediately. Mindcrank triggers when an opponent loses life, causing them to put that many cards from the top of their library in to their graveyard. Syr Konrad, the Grim triggers whenever a creature is put into a graveyard. After these two cards cause each other to trigger enough times, the game stalls for a moment, then resets back to the beginning and starts to replay the whole game. When it hits the point where it reset, it starts over again.