How to reproduce: Have the X-cost card in hand, play against the 37-boss and play 3-cost and 37-cost cards until you have 0 energy, then observe that the tooltip of the X-cost card becomes a mess (screenshot) incl. NaN values. When this card is played, the screen eventually goes white in a visible and audible explosion, as if this behavior was intended, but afterwards the game becomes unresponsive until I refresh the browser with F5.