If you make a play that is sufficient to kill your opponent the GUI will ignore the conditions/results of any subsequent play in that chapter and show every subsequent card to be playable without issue.
So If your opponent heals and you don't kill them it is possible to kill youself on a creature / burn a support because you don't have enough gold / fail to activate a certain condition without any warning because the GUI ignores everything after the point in which it thinks you have won.